Is due to the fact that they don’t understand the dynamics of relationships. Especially those between two people intimately involved. Relationships hardly ever last because two people are “made for eachother”. Both sides have to work towards being close, and sacrifice here and there in order to meet at a middle ground. You shouldn’t be unhappy, but you also shouldn’t be so happy you take everything for granted. It’s a complicated balance thats hard to keep stable.
Just saying, you aren’t single because you’re fat, ugly, tall, short, or anything physical. If you want a boyfriend, it’s literally as simple as asking a guy. However, keeping a boyfriend around - thats where the critical thinking and communication has to happen. Without it, things will not last.