Most people fall in love more than once in the U.S.
Generally speaking the (first) time we fall in love is while we're still teenagers attending high school or possibly while in college. Oftentimes at that point in one's life they have not figured out who they are, what they want, or need in a mate for life.
It's also a time when hormones are raging and having no real responsibilities like paying mortgages, car notes, dealing with a demanding boss, or raising children. Many people look back at their first love with unrealistic romanticized thoughts when they compare it to their present day love.
The reality is when one has less demands on their life they naturally are happier than when they have a lot to deal with. Real love involves making time to put in the effort maintain a relationship. Students only had class homework and the rest of their time could be used to spend with or talk with their mates about their "future" together.
Lastly life is an evolutionary process. By that I mean whatever was on your "must have" list for mate at one stage of your life is very likely to change. At age 16 all you may have wanted was an attractive, popular, or funny mate. For most people that alone will not cut it when they reach their mid 20s and early 30s. They date a completely different "type" of person.
There are over 7 Billion people on this planet! Odds are there is more than (one) person who shares your same values, wants the same things for a relationship, naturally agrees with you on how to obtain those things and last but not least the two of you will have a mutual depth of love and desire for one another.
If a relationship/marriage fails it makes no sense to "give up" on finding a new love. This is why people fall in love and marry again.