It's a combination of immaturity, raging hormones, and mistakenly confusing "infatuation" with love. The recipe for "falling in love" as a teen is very simple. Meet a "hot looking" guy/girl who is (fun) to be around and shows a personal interest in you by saying a few sweet things. If there is sex and discussions about being together in the future they'll believe they met their "soul-mate"!
Adults understand in hindsight that most of us (fail our way to success) when it comes to love and relationships. If this were not true we'd all be married to our high school sweethearts!
It's practically impossible for a teenager to have figured out (who they are) let alone know what they want and need in a mate for life!
Therefore they allow "impulsive connections" and "happenstance" to dictate their relationship choices. They're unaware of "red flags".
When I was in Jr. high a 13 year old girl attempted suicide because her 14 year old boyfriend broke up with her in order to date the new girl at the school. I would suspect after she had gone to college, started a career, gotten married, and had children...etc She most likely looks back at that time and wonders how in the world did she think life was not worth living because some 14 year old boy was behaving like a 14 year old boy!
Teens lack a "big picture" perspective with regard to life. They can only see or think about (the immediate) for themselves. We're always evolving. No 13 year old should be thinking that life isn't worth living and no 14 year old should be held responsible for the well being of another person. Teen dating is about fun and exploring/learning.
The perfect guy/girl for you at age 15 or 16 may be completely different from what you want at age 25 or 35. Simply being cute and funny is not going to cut it in the adult world of love!
It's also easier to "fall in love" when a person has no demands on their life. Teens have parents to provide a roof over their heads, keep the lights on, food in the refrigerator, clothes on their back, and give them money to spend. There is no demanding boss or other serious issues that require dividing your mindshare. Who couldn't fall in love if they had (no responsibilities) and nothing to worry about?
It's having to deal with real world challenges that will determine if someone is right for you. How do they react to stress and can they balance life. Adult couples "in love" don't live in a cocoon.