Lighten up

I wish he'd lighten up a bit and realize that we were only joking.

The mood at work has been tense, so he's looking for ways to lighten it up.

Sondheim's sense of life — which is pretty pessimistic (I don't think that anyone ever ends happily ever after in a Sondheim musical) — is brought through without trying to lighten it up.

you lighten up my heart when I start to cry

Near-synonym: slim down

He can't get any more power out of it [his drag racer], so now he's looking for ways to lighten it up.