When it boils down to it, all social media is attention seeking of some kind.
I don't do Insta because I never caught onto it, so it may be different, but every part of an interaction on Facebook, for instance, is a form of attention seeking. There is nothing wrong with that at all, by the way. I like sharing in people's lives that way and seeing what they are up to.
So I 'like' pictures my friends post of things they have achieved 'here's me up a mountain', 'here's a cake I made' and the 'like' in that case is appreciation of the thing they have accomplished or because they took a good picture, or their DC looks adorable...etc.
Sometimes it is 'here is a bunch of us all dressed up on a night out' and I might post something about how good they are all looking and how they look like they had fun.
What I find more problematic are the selfie posers. Especially when dressed with tits out and a duck face - Because I wonder what they are wanting the attention for. It isn't usually documenting a day/evening out or showing off something they've done. It is 'here is me!' and it seems most likely that the attention that are after is: 'tell me how hot I look'
In that case, a 'like' is affirming that. 'Yes. You look hot'
And I might do that for a female friend if I thought they were posting because they were feeling insecure and wanted that sort of validation...
But what does it say about the random (or vague acquaintance) man who 'likes' it. It does suggest (to me) 'yes. You look hot' - and will be read by the poster that way too, if that's the validation they were after.
That was rambly and probably made no sense. I'm also single, so haven't had the problem with a DP liking any women's selfie's. I'm not sure I'd love it, though - for the reason I've badly described...