This is a bit of a specific context, but I think when Crux is just getting adjusted to being his "new" self after waking up from his coma, he has a lot of depressed concern about himself. Who was he before? Why do so many people fear him? And fitting that, I just think of the song Be Honest by Forrest. https://www.youtube.com/watch?v=Zo1WG4FVl2k&list=PLl4nvk5PTgHmejcBrUwcscOp_VG543WYA&index=10

For Valerian, this is a little more sappy but I think when he's gone awhile without seeing Tourniquet and really misses her, the song Being Human fits his mood (https://www.youtube.com/watch?v=RZ8y37jMX4A&list=PLl4nvk5PTgHmj-1T4atrSPKST5z7Xt5RU)

It's not in a literal sense, he has no regrets with loving Tourniquet for exactly who she is, but the way that the song talks about just taking time to go somewhere and live in the moment. He knows he's not gonna be around as long as she will, and he wants to take the time he knows he has to give her those moments to remember fondly.