The theme song for Seasons of Love is "Little Something" by Mag Lam.
I'm not a fan of Mag, but I have to admit this song sounds very nice.
The song used for the spring season (Him Law & Toby Leung) is "Onion" (洋蔥) by Taiwanese singer, Aska Yang.
The song sung by Myolie Wu in the winter season is "I'd Love You to Want Me", originally by American singer Lobo.
Read Overview - Seasons of Love