I have always trained on my own...till I married DH. He doesn't like me riding alone, he worries about the traffic and such, I laugh and tell him I rode my bike, alone, on patrol at 2AM (this info doesn't make him feel better).
Now if he can't get away from work, I ride alone. I prefer alone. I put ONE ipod bud in my ear (he also hates this) and spin away. I have done lots of miles this way for many years.
I don't MTB alone. The potential for me to be upside down in some creek with no cell reception is pretty high.