Beebe, Beebe and Ivy, in their book Communication: Principle for a Lifetime, there is mention of two important concepts: 1) relationships of circumstance and 2) relationships of choice. Like so many things in life both concepts may be present in any situation, but it pays us to note the strengths and weaknesses of both.
Relationships of circumstance are common and may be very important. For example, two students are taking essentially the same courses in college that lead to the same professional capabilities. They actually take some of those classes together on a regular basis. Each notes the others existence, but nothing special catches their attention. Both are busy and have little time for any distractions. But as the year moves along each notices that they enjoy group class activities that includes the other.
After a while they agree that it would be mutually beneficial if they worked together in one of their classes. Each slowly became aware that when the other couldn't make it to the work sessions there seemed to be something missing from their personal as well as their professional life. They found that there were multiple levels in meaning of the phrase, "I missed you."
These two have found the strength of relationships of circumstance. They found someone with shared interests and goals with whom they could feel comfortable. This sort of relationship could easily move into something long-term and close. Good: because it builds on the person (brain) of the other and that tends to be less transitory. Bad: because over a life time you will probably be working closely with a number of people (brains.) One must be ready to make personal ethical judgments about these relationships.
Relationships of choice are different. You don't simply become aware of the other, you actually seek the other out. There are inherent risks here. Should you be feeling very lonely, you might choose a person who will simply fill the void. Once you have rid yourself of the feeling of loneliness you may no longer see any particular reason to maintain the relationship.
Another possibility is that you seek someone out because it would be a "feather in your cap" if they were willing to be seen with and spend time with you. You didn't really give two hoots about them as individuals (brains) but instead used them to prove your "worth" to others. This clearly involves personal ethical decisions. People aren't objects to be collected, stored, given or thrown away.
An additional risk is that you visualize yourself "living forever after" with someone of great personal attractiveness. If and when they lose their attractiveness there may not seem to be sufficient reason to maintain the relationship. It is likely that there are a couple of considerations in establishing such a relationship: "I'll be able to reshape them into what I need," or "I can put up with what they lack, say or do because they are so physically attractive." Neither of those ideas hold up when you are dealing with a significant other. That would be a recipe for pain.
It should be clear that I think some of the better long-term relationships are built on relationships of circumstance rather than choice. Both circumstance and choice are involved at some level in most choices. Thinking about the entire process in advance might not be natural to humans, but it might very well save us all some grief. Decisions about relationships are complicated and merits our full attention.