One of my good friends just got into a relationship and it sent me into a bit of a panic attack!
Why did you have a panic attack when you find out your friend just got into a relationship?
First I felt jealous, but then I felt happy for him because he deserves someone who will love him.
It's good to be happy because your friend is in a relationship with someone who will love him. I think you should tell him how pleased you are for him, because he'd probably like to hear that.
I should tell him so he doesn't feel lonely during the relationship.
That's good and nice of you to care about your friend even when he is in a relationship. Caring about those you are close with is always a nice feeling.