Maybe it's their goofy smile; maybe it's their razor-sharp wit; or maybe it's simply that they're easy to be around. You just like them.
But scientists generally aren't satisfied with answers like that, and they've spent years trying to pinpoint the exact factors that draw one person to another.
Below, we've rounded up some of their most intriguing findings. Read on for insights that will cast your current friendships in a new light - and will help you form better relationships, faster.