While never giving up is a key trait of most Marvel heroes, there are occasionally reasons that iconic characters like Iron Man and Thor have to move on to the next threat quickly. They could have a short attention span, be jaded by the industry they work in, or simply have too much on their plate to focus on one specific issue obsessively.
These heroes thus mean well with their actions but have a habit of dropping a case when it gets difficult in order to spend their valuable time and resources elsewhere. Eventually, they loop back around, but they can oftentimes get more done via this approach.
10 Iron Man
Tony Stark's mind works much faster than most. He is a futurist in principle and that means working at a speed that stops him from getting slowed down by specific problems. He would rather find other solutions. There is an obsessive side to his personality, but he is trying to combat that.
As one of the richest superheroes, Iron Man can simply afford to give up, his resources always working on new leads in the background. It's therefore a matter of attention and emotional sturdiness. Defeat can weigh heavily on Stark, and he has chosen to keep moving forward to other scenarios he can fix, giving up and moving on quickly from those he cannot.
9 Nick Fury
Nick Fury might be one of Marvel's busiest heroes. The character was previously the head of S.H.I.E.L.D. and has taken it upon himself to watch over Earth-616, getting ahead of all threats that may appear. He is stretched thin across countless cosmic, supernatural, and villainous cases.
Nick Fury has to give up quickly, in order to maintain a level of control over every aspect of his job. He is good at delegation, occasionally passing on his information to other heroes, so that he can continue on another project. Many forget that Fury is a genius and thus knows how to best operate with the time available.
8 Ant-Man
Hank Pym has often been erratic. His personal problems, alongside the complicated villains he has to face, have resulted in the character often running away from his issues. He doesn't always face up to key foes, or his past mistakes, instead putting his energy elsewhere.
Projects like Avengers Academy for example are instances where Pym wants to forget the past, and channel his work into something new. With villains like Ultron, who Pym guiltily created, he has become jaded. The constant cycle of repeated threats means that he never truly believed he could defeat his nemesis.
7 Doctor Strange
Just like Nick Fury, Doctor Strange holds a very special protective role in the Marvel Universe. The Death of Doctor Strange arc demonstrated with great conviction what happens if the Sorcerer Supreme is removed from the landscape. All hell breaks loose.
Strange has built magical teams before, to continue to contend with threats, but the truth is that there is too much to get on with. Strange thus has to give up easily and keep forging a path forward. Supernatural enemies can come from all angles and to briefly subdue them is enough to keep the Earth spinning. The inevitable is they will forever keep on attacking.
6 Captain Marvel
Captain Marvel is a valued team leader with a military background who understands the need to prioritize. She's responsible for groups like the Avengers, while still providing significant support across space, overseeing the protection of countless planets with alien allies.
It's been drilled into her through training, that she cannot get emotionally invested in any one threat, and has to divert attention where it is most needed. Captain Marvel will have to give up easily; with the whole universe under her watchful eye, it would be impossible to chase everything up in such severe detail.
5 Professor X
Professor X is a leading member of the X-Men and Krakoa. He is too important to watch over everything. Much like Nick Fury, Charles gives up easier because he can delegate a particular issue to someone else to solve. But Professor X has a different kind of obsession.
If an issue's resolution does not benefit himself or his goal for mutantkind, he has been known to abandon it. As Krakoa has grown, he has become more reckless and secretive in those pursuits. He is more likely to give up because his interests now force him to focus on more specific areas.
4 Quicksilver
Quicksilver has a short attention span and an even shorter temper. He can hold a very personal grudge, but if something isn't affecting him emotionally, he will drop it quickly. He gives up rapidly simply because of that fleeting interest, his super speed affecting the way he thinks.
If a villain wronged his family, for example, then he would be absolutely obsessive in his quest to find justice. The morally gray hero is a little more erratic in other instances though, and it's sometimes hard to tell if he can be relied upon as an ally.
3 Storm
Storm is one of Marvel's best-written female characters. That complexity is why she might be deemed to give up easily. She has been a monarch, a Krakoan Council member, a leader of Arakko, and a prominent member of the X-Men. Throughout all these roles she has learned that she cannot win every battle.
With her interests constantly in flux, as to where her loyalties lie, she understands that occasionally she has to give up on an enemy or a mission, to pursue something else. She is not one to have any doubt in herself, but she is capable of making the tough choices after years at the top.
2 Thor
Thor has a lot of self-doubts. He has even spent some time abandoning the name, instead calling himself Odinson because he feared he was unworthy. There are thus two reasons why Thor gives up easily. The first is as King of Asgard, there are too many duties to forever pursue each and every threat.
Secondly, that notion of worthiness plays on his mind too much. Thor sees himself as unworthy in so many situations, that he feels his allies might more productively complete the job. Whether it's Jane Foster becoming Thor, or a range of other Asgardians watching over his home, Thor's character arc is fueled by giving up due to confidence.
1 Black Widow
Black Widow has been in the game too long to hold grudges like other heroes might. She has been jaded by the whole process and understands that there isn't always a clean ending. Sometimes she has to give up on a target, impossible odds stacking up.
For her, it's all about being able to adapt to a situation and knowing when it's tactically the best option to attempt a different path. She's no less heroic, but she's realistic. She has learned these tough lessons the hard way and continues to operate as a vigilante through her own, experienced methods.