To love someone is to make that person part of our life.
Yet, the object of our love is contingent and not necessarily so. I may love Sophia (not a specific person, name means wisdom), but if Sophia had never existed or had I met someone else, I would love another person instead.
We cannot necessarily love because the condition of free will would not be present for love to be possible. The object of our love has to be contingent as love is an act of the will to choose that contingent person.
But this is precisely what makes love so beautiful – to say that I love you (as my beloved or as my friend) means that even though you are a contingent part of my life, I nonetheless want you to be a significant part of my life as if you are necessarily part of it. Someone else could have taken your place as my beloved or my friend, but it is you whom I have chosen to be my beloved, my friend.