I lift alone because I grew up lifting alone. My first year of training would have been annoying for a partner because every two months I came up with what I thought was a radical new program.
Lifting alone has its pros. I love having my log book in the gym, a little hand sized book. To me it is like a scientific experiment, in which I manipulate variables and push hard to make it to the reps/weight I want. I have pretty immense focus in the gym and thats how I hit new PRs. It would be hard to have a partner when I have to make my own adjustments, when I suddenly need to go light et.c.
I have been told I have a natural talent to push people beyond old limits, and make a real psycho environment. When I first watched dorian yates blood and guts, I thought damn dorians partner must be my double in life. For me it just doesnt work, a person and their words wont make me lift harder, but others say I am great at getting the killer intensity for them and it is needed every now and then. If this works for you then you benefit greatly from a partner.