Show, don't tell.

Show:

Suddenly he found himself smiling more, and occasionally he even laughed. His sarcasm withered away and instead, what took root was an incredible earnestness to explain his thoughts and feelings to other people and even listen- no matter how stupid he thought them previously. Eventually he figured that this odd happiness couldn't be just a coincidence: it was sustained by the way she dotted her i's with little hearts whenever she wrote his name.

Tell:

He was in love.