I do most of my riding alone. Being self (un)employed I can just nip out for a ride during the day when the opportunity arises, so easy to do that on my own. I can go at my own pace, work as hard as I like without either waiting for people or feeling like I'm holding others up, repeat things a few times if I fancy, go exploring, or just go and sit by a lake for a change. Plus I'm no riding demon so when I'm on my own I can (if I want to) at least feel like I'm shredding the ragged edge of the gnar without seeing someone else leave me for dead without even trying. As for motivation it's easy - will I feel better when I get back from a ride than I do right now? That's always a yes, even if it's just a ride down to the shops for some chocolate.
However when I ride with a group I find that I tend to try more stuff that's just at the edge of my comfort zone - not sure if that's peer pressure or just being sure that there's someone to scrape me off the ground/tree/rocks when I cock it up. I fall off a lot more when I'm riding with other people.