Either romantically or as friends?
For example, all my working life (30+ years) I have worked in very small close teams, where you work in quite high stress environments with one or two other people. The same people everyday until one leaves their job, so you get to know each other quite well. Because of the industry, most of my "partners" have been men.
In all that time, I've worked with people, who on the whole have been good decent people, professional, supportive, good company. I've only ever had one I really didn't like and even then, once you got to know him he wasn't that bad.
Whilst we had good working relationships and even had fun at work sometimes, they never became friends outside of work and there was never any hint of romance/anything sexual.....except for one.
Over a period of 4 years we developed a close friendship, saw each other outside work, spent time with our families and t almost certainly became an emotional affair and very nearly became physical. At that point we put a stop to it all, which was really hard.
Looking back I can't see why he was different to any of the others. He wasn't particularly attractive, he was "nice" and funny, but not more so than many others. I don't think he set out to target me or to have an affair and I certainly didn't. In fact the only thing I can see that was a bit different was that he was a really committed family man, maybe I let my guard down, feeling safe with him. So why did this "happen" with him and none of the other people I met?
Similarly with friendships. I've belonged to a running club for nearly 20 years. Over that time I've met a lot of people and made some really good friends, but really only a handful of the 100s of people who've passed through the club have become good friends rather than club mates iyswim. Why is that and why them (or me for them?)