Star Wars: The Old Republic gives you a companion for your leveling adventures, but there's no need to have a real person with you when completing the game's solo content. World of Warcraft is tuned so that everyone can level through quests solo, even if sometimes you make it through by healing yourself until monsters give up and die of boredom. It's a welcome change from logging on, finding out your friends are offline, and giving up for the evening.
But let's say you have a friend who will play alongside you if you want. On the one hand, you can suddenly make your life much easier by pairing up to tackle content designed for just one, doubling what you can accomplish. On the other hand, there are always quests that become more frustrating with two people after the same objectives, even when those two people are ostensibly working together. Plus, working alone means never having to stop for five minutes because your partner has to put a squalling baby to bed. Given the choice, would you rather be grouped up or solo when taking on content that allows for soloing?