The reasons vary. I do have a pretty fulfilling job, interesting research, engaging hobbies.

Thoughts that stop me when I might actually do something:

1. I've come this far, why stop voluntarily now? Seems like a waste.
2. Fearing that at the last moment, just past the point of no return, I'll regret my choice. I don't want to die with that as my last thought.
3. To Catholics, suicide's a mortal sin.

Eta: rereading your post above, it sounds like your life is kind of insular - family, fiance, dogs, doctors. Perhaps volunteering would help, or eventually a job, even part-time, if that's possible?