...That is waking up to the recognition of 'what might have been', years later.
Suddenly understanding that you have been operating on the expectation that the other has never had the same intentions as yourself is a great kick in the teeth.
"I love you" can mean anything, including that "I love you and need you, the best I have ever known, even though I can't satisfy, or understand, your real needs or hopes."
Good judgement is best, beforehand.
Blessed are the parents who know and teach good judgement...early.