Why is it more meaningful when a stranger compliments you?

To me, it's because this person knows nothing about you but has judged you positively and forms a surreal, strong opinion of you and feels compelled enough to compliment you in that short amount of time they've been aware of your presence. It's happened to me twice or so and I always feel really overwhelmed.

Yes, I think that they mean more because they are quickly formed and in the moment of whatever situation. We live in a world where it's uncommon for people who don't know each other to be positive towards one another so whenever that trend is broken, I feel happy about society and within myself, even if it's just for a short amount of time.

I also feel we get accustomed to receiving compliments from friends but especially family because their love is unconditional and therefore they feel the need to flatter you and show their attention with compliments but there is of course love and meaning behind those compliments nonetheless.

For me, any compliment is always an honour to receive but only if I feel there is real passion and authenticity behind it.

i reckon it's because we feel that if it's a stranger who compliments us, then they don't know anything about us previously, they are not there to say something nice, (they don't have to speak to you at all) but they still choose to express their positive feeling towards you.

I think that is why we feel like it's extra special.

I agree. I think close ones feel obligated where as strangers have no ties to you and yet they are still willing to speak up and say something nice :)