Why is my boyfriend “tired” and less talkative when around me but not others?

Hello,

we’ve been together for almost 2 years now, he’s 17 and I’m 18.

i noticed when we are around a group of friends or with another person, he’s pretty energetic and loving with me lots, doesn’t ever complain that he’s tired, and is usually able to joke around and keep a good conversation.

when it’s me and him alone, a lot of times he’ll complain that he’s really tired, won’t be very energetic and is hard to talk to, he seems down. This is usually after we are walking back to our cars after a long work day, or if he worked and we are hanging out after.

it seems like he can stay in large groups longer if I’m not there, but wants to leave to be alone together fairly quickly and gets “tired” very fast. I just don’t know, he’s still very affectionate, tells me he loves me lots, and is so sweet when he’s “tired” around me he’s just hard to talk to.

My friends said maybe he acts how he truly is around me because he’s comfortable but I don’t know.

what is this?
Why is my boyfriend “tired” and less talkative when around me but not others?
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