Well every one of us have different experiences in lives, many would have fallen in love multiple times and each time they must have felt as if its the true love and then life moves on to another point in time.
I presume we are talking about True Love, not the love which a boy says to a girl after every date. Love has many definitions but True Love has none.
True love is like when one is touched upon by some thing beyond explanation as if it has come from another place not found on this planet, as if the true love itself is a live person embedded in the two lovers, like two souls one body or one soul split into two like a twin flame.
True love if happens, can only happen once in a life time and yes more likely true love hits you when you are already in love with another person, its ironic but those who experienced True Love knows what I am trying to explain, though I feel I can't ever.
Love is like living your life/duties with the one you think he/she loves you, and True Love? well don't search for it, it will find you when you least expect it and when it does, you are never same again.
In the end, one who experienced True Love is the most lucky person in the whole universe and he/she should embrace it and not deny it ever.