Evolution can change just about anything, except for one: accepting single individuals as happy people. While I admit that there is someone out there for everyone, I don't see the point in pressuring people to actually spend every waking second of their lives to find that someone.
The truth is that some people just don't want to bother themselves with it. The whole 'regret phase' people keep talking about, when you'll wake up one day and realise you're still single. These people wake up every day and are happy that they're still single! Here are some of the reasons why:
1. Their utmost priority isn't finding a partner:
Not everyone thinks that happiness can solely be achieved by being in a relationship. It's honestly about priorities, maybe they have a job and don't feel like risking it in order to be with someone in another state whom they might or might not end up with forever. There is also a possibility that they'll come to regret their decision every time their relationship faces turbulence.
There is always a risk when it comes to relationships and these people don't really want to go through that risk. They define happiness with so many other things such as their career or their families etc.