How do you handle loneliness as a solo hacker?

• Get a coworking space
• Partner with another freelancer to do projects together
• Get a second part time / volunteering job, for example teaching kids to code

How about travel to some countries with cheap living expense, hook into some co-working space occasionally, and do some short term travel here and there?

Find a soulmate

• Find someone to work with, similar to co-founder
• Attend local events; we go to lunch and learns, hackathons, etc. -- we do these to interact with potential clients, but they would totally work for social interaction as well
• Sign up for a course or workshop, or run one yourself

The main theme is you should find some local, organized activities of interest; preferably ones that repeat weekly.

The thing that makes the biggest difference for me is having a co-founder, so if I had to choose one, that would definitely be it.

I have the same issue. This is actually a big part of my motivation to start a business in the first place. As I see it, there's not really enough time to spend time with family, spend time with friends, attend to life's obligations, AND have a job.