According to The Don’t Marry a Jerk Paradigm, the following four areas need to be in balance. You should find the following qualities in the other person in equal amount. If one is high but the other is low, you will have problems.
You can trust the person.
The person is committed to you or has the ability to commit if the relationship develops.
You can rely on the person.
You know the person well (you have spent enough time with him or her to know then well).
Touch (have intimacy to the degree the above exist).
When a person has a bad relationship, they usually say:
I should have known better, meaning I didn’t know how to determine who is a fit person for me.
I knew it wouldn’t work but overlooked what I didn’t like, meaning the heart overwhelmed the head.
There needs to be a balance between head and heart.