I trained alone for many years. Anytime I can train with my brother, it's far more productive. Kinda like an EF meet up or visit, I guess.
I've had a few good training partners over the years, and that always kept me going, but the majority of the time, I'm a loner, too.
Now, I basically train alone except for squat days, when I try to hook up with a couple of the guys I work with.