There've been lots of people who've been extolling the virtues of being single to me recently. Still, I can think of three things offhand that - to me - are strong enough reasons to want a companion:

1. You know there's someone who you mean something to, who notices when you're not around.
2. (Applicable to me only) I really do need someone to help balance out my ultra-critical side, because I am very very harsh on myself. I've been trying to ease up but there're days I slip.
3. It's nice to have someone who'll give you all the hugs and cuddles you need without you needing to ask.