One of my best friends has a nasty habit of doing this constantly. He views everything as a competition and I think he has issues with appearing inferior or inadequate so he always has to put others down. For example, I recently moved into a much bigger flat with a big balcony, so I bought a table, chairs and a barbecue and invited him round to hang out and chill. Instead of just relaxing and enjoying each others' company he goes off on one about how what I've bought is s**t, I quote "you buy s**t you get s**t" and how I should have spent 4 times the amount on the stuff that he'd buy if he had the space for it. This has been a constant point of friction between us because to me it's irrelevant what brand or how expensive something is, as long as I can use it and enjoy it with those closest to me, whereas he has to have a big flash Audi, iPhone, expensive clothes and furniture etc. I can really wind him up by telling him Madonna wrote "material girl" just for him and he gets really annoyed when I sing or hum bits of it.