A honest smile

Izuku Midoriya was forced to show fake emotions almost every day of his life, starting from the childhood. No friends, no family, except his mother, that he loved very much. Basically no people he could share his burden. Hell, most people just wish his death. And for what? Because he is not like they are. Being born like not everyone else can be very scarry. There was almost no happy events in his life, except the times, when he was celebrating something with his mother. It looked like, that Izuku also unconsciously keeped her "locked". Since he was 4, she never really was hanging out with her friends. They both were basically alone.

Momo Yaoyorozu also was forced to out on a "mask" on her face. Being born in the rich family means you have to meet requirements of such family. You have to speak in a rich manner, always do everything by rules and basically no personal freedom. While most children were playing somewhere on the playground, Momo was forced to study and follow the rules of her parents. Day after day. Maybe it was because of her quirk? Sometimes she really hated her quirk, because it was setting a very high limit to her. She couldn't do what others do. And unlike our previous character, she was basically alone even though she was surrounded by a lot of people.

I guess... When you have a person that is just like you, it's a pretty good reason to smile...