Dean:
Sam. He was so annoyed with both of you at the time when you refused to confess you feelings for each other and when you finally kissed he jumped up shouting "FINALLY!" and running out if the room whooping.
Sam:
Dean. He often teases you about your relationship, but he's secretly just happy Sam finally found someone to love - who's alive.
Castiel:
Dean. Seeing his best friend with a girl he deserves makes him happy. He also tries to teach Cas a little bit about relationship since he's such a newbee.