I think there's a balance here, and cultivating some amount of loneliness -- not solitude -- is also good. For example, two people in my life are "Alice" and "Bob", who have been married for decades. Alice once told me that one of Bob's great weaknesses was that he'd been alone for too long in his teens and 20s. This had benefits: Bob's social needs are low to non-existent, and while he has loving family and friends, he enjoys sustained solitude. For example, weeks of overseas travel where nobody knows him and he doesn't talk much are great fun for him.
The downside was that he got too used to this. Engaging with other people is messy and hard to control. If you don't have that gnawing bit inside that's hungry for attention and connection, it's easy to decide all that mess isn't worth it. That can hurt people who need those connections more, as they may feel themselves weak or pathetic by comparison. Relationships with other people can become strained if you are at least approximately self-sufficient.
Briefly, my point is most people need (or at least want) to be needed. Somebody who's truly content with solitude isn't going to put that need out there, so relationships with such a person might be more difficult. Of course, too much need is another problem. But a little need seems good.
(This point is pretty tailored to the HN community, which appears to have a lot of stoicism and solitude devotees. I like both too, but I think the point above is worth remembering. The mutual exchange of vulnerabilities is an important component of close friendships.)