Jack:
He got hurt and you were there for him while he was in pain. He was completely vulnerable in front of you and you took care of him, that's important to him.
David:
When he found out you had stolen one of his books from school so you could help him with his homework. He was so touched that he just randomly ran up and kiss you.
Crutchie:
Every time you smile. He falls in love with you even more when you smile.
Spot:
You two had had an explosive fight which ended with you on crying and on your way to Manhattan to stay with Race and Spot hiding from the other boys and crying (yes, Spot does cry, he has feelings) and feeling horrible for yelling at you. That's when he realized he couldn't live without you.
Racetrack:
When you fell off Tyrant and almost died. He was so scared of losing you it almost drove him insane.
Blink:
When the Delancys attacked you two and you helped fight then off. He loved it.
Mush:
He realized he loved you when you danced with him. It's pretty simple, even though you had no idea how to you tried anyway.