(I ship it, so this is just my headcanon for this couple.)

Romano tells her he loves her frequently and makes her happy.

Monica helps calm him down when there's a thunderstorm and he's scared or when his social anxiety gets the better of him.

However, Romano feels he should be able to do more for her, he fails to see how happy he makes her. He always thinks he isn't good enough for her. Just the way his mind works.