There have been many young people who have found their partner left because they acted immature, and they often believe it is the former partner’s fault for being too picky. They might have seen the end of the relationship as the other person’s problem, but they might never quite get over them. Their future life could be filled with many relationships that end quickly, but they could finally mature when they realize they are the problem in each case.
There are few people willing to wait years for someone to grow into adulthood, and many of those former partners will move on to good relationships. They will find someone who understands their value within a relationship, and they will not have to put up with the bad behavior they experienced with the person who chose to be childish. Many of them will be unavailable when their former partner finally grows up, so that person will generally have to find someone new.
It often appears that there are a limited number of people to date in any area, but modern mobility does offer new opportunities. For those who managed to wreck their chances of a good relationship due to lack of maturity, finding those few new people in a local area might be difficult. If they are very lucky, they will be able to meet them quickly once friends and family realize they have finally matured. Others might be unwilling to introduce them if they are unable to prove they have really managed to live life as an adult.
While it is possible they could reconnect with a former partner, it is very unlikely. Many people who were immature in a former relationship hurt the other person with their behavior, and it can be difficult to overcome that reputation. If their luck is truly good, they might just find the person they loved and lost is single and ready for a good relationship with them.