Usually they happen by having a broad social network to begin with, and by focusing on friendships as the stepping stones for a relationship to happen. Having existing friendships allows you to practice and refine your social skills in preparation for a relationship, as well as increase your exposure to people, lessening your anxiety.
I made the mistake of dating guys at a time when I didn't have a lot of friends and when my social skills weren't the best. They led me to pick some really crappy guys (two who wanted to become criminals when they grew up (!) who didn't necessarily have anything in common with me or that could share things with me. This was because I was focused on getting with any guy who wold pay attention to me rather than picking someone I could be relaxed around, have fun with, and be myself with.
Even if you want to take a more disconnected route and meet someone online, establishing a network of friends (even if it's only 2 or 3) will give you some practice and confidence when communicating with a potential partner. Also, this network can benefit you as they can introduce you to more friends: and potential partners as well!
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Given a “tentative” diagnosis as a child as I needed services at school for what was later correctly discovered to be a major anxiety disorder.
This misdiagnosis caused me significant stress, which lessened upon finding out the truth about myself from my current and past long-term therapists - that I am an anxious and highly sensitive person but do not have an autism spectrum disorder.
My diagnoses - social anxiety disorder and obsessive-compulsive disorder.
I’m no longer involved with the ASD world.