Rachael said:
from my personal experience, intros don't always work.
Your personal experience is very limited. And I don't think you've tried everything to make intros work. Do you neuter them when it doesn't work?
I would never allow a rat to live alone when a neuter almost always works in making them more accepting.
Then there's getting new buddies to find the right match, or changing the cage, or changing the area of the cage, or making sure that rat is not around any other rat while attempting intros, or the last approach, rehoming the rat so that a new home can provide him buddies. Sometimes homes with established colonies can freak a rat so bad that a new home is needed. I highly doubt that you've done all this with your intros.
It's really quite easy for anyone to try once and think it doesn't work so little guy must want to be alone. That's BS! You have to keep trying, you have to try different methods, you have to neuter, you have to do everything possible to help your little guy realize that yes, a buddy is much appreciated. I've had some real aggressive rats, Radar would actually jump and attack another male and this was after his neuter. I found him a docile but yet strong girl to guide him in how to be a rat and learn rat body language. After a few weeks with Isabelle, Radar was ready for intros with the colony again... he enjoyed the rest of his life surrounded by furry buddies, he even bonded strongly with with them.
If you've done all those things and still the rat is too neurotic to accept buddies, then you allow him/her to live alone and be his/her companion for at least 12 hours in the day... they need companionship. You need to learn to gently massage them with your mouth, you need to keep them close to you and breathe your hot air on them, they need to hear your heartbeat and smell your scent, hear your soothing voice at all times.
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