Many people look for familiarity in relationships. These are the same people whose relationships keep failing. Someone (I think the internet says Einstein, but there are multiple sources listed and I'm WAY too lazy to date the origin back) once said, "The definition of insanity is doing the same thing over and over again, but expecting different results." This is a common pattern in some relationships.
There are two ends of the dating spectrum in our generation's mind: You're either terrified of being alone/dying alone/etc. or you completely reject the idea of a relationship. Some people float back and forth between these two and there's a select handful that falls into the gray area. Even though there are two ends of this spectrum, we're going to focus on the people who are terrified of being alone. If you're looking for a reason the other portion of the population drifts toward rejection of relationships, feel free to read this article I wrote about the fear of missing out on things in dating.
When people are afraid of being alone, they cling to anyone they can -- friends, boyfriends, girlfriends, etc. These people will also cling to relationships that are not necessarily bad, but it isn't what they really want or need. When a person is afraid of being alone, they will stay in a relationship, not because it makes them happy or it provides any other benefit, but because they are afraid that is all they will ever have. They confuse being comfortable with being in love. "Routine" or "fine" are no way to describe a relationship.
If you've read the FOMO article and now you're reading this one, you may be asking, "Didn't you tell us just last week that settling in a relationship is good?" Yes, settling is good when you are happy in the relationship. It is good when you have a special connection with someone. It's not good to settle just because you are afraid nothing better will ever come your way. It's bad when you think you deserve less than your worth. The idea of being comfortable with someone is hard to let go of. Putting ones self out there is not always the easiest thing to do. Even break ups can lead to this desire to be comfortable again. Example: the two people break up, a week passes, one party thinks, "One text won't hurt. I just want to see how she/he is." That text leads to plans, and those plans lead to that all too familiar comfortableness. The lonely party remembers how good it felt to be comfortable with someone. Ultimately, this person gets caught up in the routine and isn't happy, but they still can't let go.
Falling into habits is too easy nowadays. Falling into old habits is just as easy, if not more so. The easy route is the route that gets people lost. It's what ultimately screws people over. Just because you're comfortable does not mean you're in love. There will be the sentimental value to it, just as there is with all old things and belongings. Once you see just how liberating it is to break routine and start fresh, though, you'll wonder why it took you so long to get there in the first place.