Sounds like your friend is somewhat of a jerk. :/
Anything anyone collects is special to THEM, and that's what matters. You don't need a "theme" to your collection, that's ridiculous. If you want to collect within a theme, go for it, but thinking every has to collect just like you. . .hahaha no.
If I *had* to name the "theme" of all my collections, it would be: what I like. I buy what I like, for all of my collections, and stick to that. If I don't like something I'm not gonna waste my money to buy it. If I had to name my MLP collection's own "theme", I guess it'd be: saving baity G1s from customizers/uninformed people who would throw them away/etc. I love my G1s to be very baity, and prefer them without tails. I usually love haircuts on them too. But there are some ponies I would only want in mint condition or something, like the Art/SDCC/Fair Ponies I want all MIB, so I don't let my love of "baity" ponies restrict me.
Like others I go through collecting phases, and right now my pony collection is chilling out on hold, so to speak. I'm selling off some of my collection and focusing on buying dolls because I'm having and easier time storing, displaying, and cleaning dust (HUGE problem in my room) off of them. It makes me sad to see my ponies covered in dust and to not be able to keep up with keeping them perfectly clean. I actually need to take a chunk of my ponies off display due to it. ):