I find working with people helps to motivate me (not that I am not motivated), but it helps with setting goals and verbalizing them to someone else.

Having someone to talk to about technical issues specific to the problem you are solving also helps.

Investors won't often invest in one founder. They like it when there is two people (what if you get hit by a bus)

If you have a good co-founder you can get more work done, or they can deal with business related issues like marketing and legal stuff or do design (I am really bad at making my applications look good)