It's not instead of having a relationship because it won't cure being alone. But before having one, or before the next one. You can also do it while in one.
It is for a number of reasons, all related to avoiding to the relationship becoming dysfunctional, in particular becoming co-dependent or worse. Ideally you should be with someone because you want to be, not because you are too scared to leave.
If you don't know who you are, the other person won't help. You'll end losing yourself and just doing what they want.
Your partner is there to provide some emotional support, but they should not be viewed as a therapist.
If you are insecure you may be clingy which can stifle the other person.
If you have unresolved trauma or other issues, you can unintentionally take it out on the other person. Being inconsistent, beyond normal moods, is hard to live with. It can gone out as anger too, which may drive the person away.
Ideally you want two emotionally secure people who can form a partnership to take on life. So each person can go away knowing the other person is ok, there are no trust issues, it is all built on solid foundations.
Note this doesnt mean you have to do everything, or that you can't ask for help, or expect the other person to do their fair share. But since there will inevitably be disagreements, you need to be able to set boundaries and defend yourself.
Then you accept it and find someone who accepts you as you are not as how they think you ought to be. If you're refering to autism, then it will be harder, but not impossible. None of us should have to mould ourselves according to the expectations of others, there's lots of things people can't help and if someone is put off by those things then they shouldn't even consider a relationship with you, or you with them.
