I’ve noticed that people who are ‘together’ for a while and haven’t done anything mean, evil, or annoying to each other usually have little trouble becoming ‘friends’. If there’s a wee bit of chemistry then occasionally they can ‘fall in love’. ‘Together’ can be at school, the office, or in SL’s case IMs, favorite shops, special sims or events.
It’s a little harder for chemistry to take over in SL because you can’t tell if he smells good or has a frisky grin, but obviously that hasn’t stopped lots of folks from becoming a little closer.
So if I have a vote, being similar isn’t nearly as important to becoming friends as being ‘together’… and SL does a great job of bringing all sorts of interesting folks together.