Because they think you are flirting and want a date - or -
Because you haven't seen each other in awhile and are sharing what is important to them.
Is there a reason they should NOT mention that to you?
I don't think having a girlfriend is private business - I think you are reading way too much into it because of your own feelings.
He is assuming you find him irresistible.
They probably feel weak in your presence like they have to remind themself they already have a girlfreind.
So, my guess cause they find you attractive.
Then think that :D.
To impress you.
To let you know that they are interested in having a girlfriend.
maybe he enjoys life with his girlfriend and since you guys were having friendly a conversation, it just came to his mind. cant help it if a guy's excited about something. : )