Are you fairly new to the job?... When I was new in my current role, with some people it took a while (months in some cases) before I'd worked out their characters, accepted their different moods and then developed my own little strategies of dealing and getting along with them. Agree confronting is the best way forward (doesn't have to be agressive - just questioning) Could it be that our colleague is jealous of other attachments with people that you've made at work? If you've done your best, don't worry about it and don't start to put yourself down over it. Sometimes acceptance that you don't always click with people is best and if it really does get unbearable, get help from your Line Manager as Sara says. Good luck!