Over the years I’ve worked out both solo and with a good training partner. A poor choice of someone to work out with can destroy you. A few years ago I got to know a guy in the gym we used to have in town only after we’d observed each other during many workouts. One can determine a great deal about someone by watching them work out, and after a few months and getting to know each other, we gave working out together a try and it was phenomenal. Todd is a competitive bodybuilder, and though younger than me we were pretty similar in strength and absolutely the same in intensity: high! Our meshing lives and workout times (we have very different types of jobs) lasted a couple of great years, and now it’s an occasional weekend workout together. We remain great friends, though, and talk all the time.
A superior partner in the gym helps one push through mental barriers, to gut out those "I didn't think I could do these" reps.
Today I work out with my wife, and it's a terrific experience. She's serious in the gym, so am I, and she's a careful spotter. When I do go real heavy, though, outside of what she's comfortable with, there are always some guys around in the gym I'll let step in and spot.
All in all, though, I don't HAVE to have a training partner to attain good workouts. I'm hugely motivated to be in the gym and to work out hard, but I keep my socializing to a minimum, so sometimes a partner is good for deflecting conversation, too.