As Christopher McCandless famously concluded, “Happiness is only real when shared”. Having a partner to share the risks, difficulties, and challenges that you'll face when hitchhiking undoubtedly improves the experience – if you can find the right partner, that is.
The main advantage of hitching alone is that you can get rides slightly faster and you can travel on your own terms. You eat when you're hungry, sleep when you're tired, and give up when you've lost hope – without having to worry about someone else's needs in the process. Plus, you have the chance to get picked up in those fancy two-seater sports cars which is always fun.
However, although solo-travel is enjoyable in many ways, the real joy of hitchhiking comes when you do it with someone else. Your long-distance journeys will be much more pleasurable if you can find someone with a positive outlook who has a similar tolerance for discomfort, if only because it allows you to take it in turns to be the one who talks to the driver.