I worked with someone like your ex-friend M. I could only take her in small doses. A 12 hour shift would drive me bonkers. On the other end of the spectrum I had another co-worker who was Mr. Negative. I never heard a positive word being uttered from his lips for 25 years. Both these people had to have been extremely unhappy in their lives. Both were escaping from a reality of some form. I am usually positive but i do have times when I am not and I think it balance out over time.