I consider myself to be pretty friendly, but many people are not. If you sell products in the same category as them and they see you in their shop- they get pissed off and/or think you`re trying to steal something from them.
I have no problem if a "competitor" comes onto my sim (Im sure they do it under alts, though) and i have no problem making friends with competitors. What pisses me off is when these "friends" turn out to be biters, that just makes me even more anti-social than I already am and wary about getting into "friendships" with people here. Its fine to like someones product and stuff, but not ok to mimic their releases and/or store layout. Just be yourself.
With all of that said, I don`t think I have ever approached any "designer" and tried to talk and/or make friends with them. I pretty much keep to myself. Infact I dont really have many friends here at all, perhaps 3 that I actively talk to and hang out with.