So many variables. It really depends on who you are (your personality and individual needs and preferences), what your support network is like, whether you have a project that you're working on, how your would-be partnership affects your time, etc.

If you're 20 and stuck in a dead-end job and horny and partying all the time, maybe a partner would help control those urges and make you both lead more fulfilling lives. Or maybe you'd both accentuate each other's weaknesses and spiral down together into a hellhole of addiction and self-loathing. Who knows?

If you're 40 and relatively established in your life and career, a good partner can do wonders for keeping your life contentment high and your routine healthy. It's good to take a break from work once in a while, and when you return to it, you'll be more rested and motivated. It gives you side benefits to your work too (vacations with your partner, weekend outings, whatever) so that when your work projects aren't going well, you can at least be motivated by those side benefits.

Regardless of your age, being together with the wrong person can drastically fuck up your psyche, especially if there's a lot of drama (insecurities, paranoia) or abuse (emotional, physical, verbal) or trust issues (cheating, control issues), etc. That can be way, WAY worse than being single... trust me, there are worse things than loneliness.

In my very humble opinion as a late-30s millennial, loneliness is best tackled not by a romantic partner but by deep and fulfilling friendships in which you can share both activities and emotional connections and good conversations. That both decreases your tendency to put all your eggs in one basket (becoming codependent on a partner) and also teaches you the day-to-day and emotional framework to learn to work with, not against, your own emotions. It teaches you a bunch of coping strategies when you're feeling down (e.g. doing something fun with a friend that doesn't physically hurt you like partying does), conflict resolution skills, etc. And those same skills transfer over to a good partnership so that if and when you find a good one, you can grow that initial romantic attraction into a lasting and mutually fulfilling partnership, without being overly dependent on that other person to the point of suffocation.

I think there's a mix of conventional wisdom here, from "no man is an island" to "you can't love someone else until you love yourself". I don't think either extreme is true; we are by nature both communal and individualistic creatures, and a healthy social life combined with periods of reflective introspection can give you the best of both worlds. But the modern dating culture -- fast swipes and short stays -- are antithetical to building lasting and meaningful connections. If you can sidestep that urge to date fast and furiously for the sake of sex, and focus on building meaningful friendships and connections instead, romantic or not, IMHO you'll be a lot less lonely over the long run. The best partners are also great friends.

But that's just my perspective. Everyone is different. Some people very much so, if their genetics, culture, upbringing, personality, etc. are drastically different from mine. Some thrive on long periods of hermited productivity. Others need constant social connection. There's no one-size-fit-all approach to this, you just gotta keep testing and evaluating your ideal position on the social spectrum. If you're lucky you'll find a good balance, and with time, the physical brain development and emotional maturity that makes it all more bearable and eventually enjoyable.