[He's quiet as Keith asks that, his smile fading a bit into something more thoughtful as he studies Keith's expression. He notices the tone right away, and just as before for Keith, Shiro's question gave him a small amount of alarm and concern, so too does Shiro now feel a little... uncertain as he looks at Keith. Of course, his mind immediately flies to one specific thing when asked this question but he takes a small breath, thinking it over-- figuring that this is a hypothetical question Keith's asking in regards to something on his mind, rather than about Shiro specifically.]
... I don't know. [He admits it, quietly, and knows it's not a fully honest answer. He doesn't want to risk Keith feeling like the question was dismissed or not answered properly, so, he elaborates.] I guess... it'd really depend on how badly I wanted it.
It's not... so easy for me to forget about others. Even if a decision is for myself, it'll affect other people. [He's the leader of a team, and even excusing the universe, there are people who care about him.] Sometimes, you can outweigh what other people think. [Like in the case of going on the Kerberos mission, maybe.] If it's something I want and I know I can achieve, then yeah. I would.
But sometimes... [He trails off for a moment, looking at Keith again, unable to disguise for a moment the longing that creeps into his expression before he schools it back into a more neutral expression.] If what you want is... Sometimes, what you want can't outweigh what's best for someone else.