Love is different from "being in love." We actually need a new word for "being in love" (an updated vocabulary, as Jacque Fresco proposed).
To be in love with someone is indeed a mental illness, an addiction. Being in love is selfish, egotistical and obsessive. It is about your needs above anything else. You need the other person in your life, just like an addict needs his drugs.
The truth is, the human brain gets easily addicted to the things that it likes: sugar, alcohol, tobacco, coffee, sex, prescription and illegal drugs and even other people.
The drug of those who are "in love" is another person, but they treat that person like an object, not a real human being, with feelings and needs. They want their fix, and they go to great extents to get it.
Love, on the other hand, is quite the opposite. To love someone is a selfless act, putting the needs of the other person above yours. That's why you can love an animal, for example, but you cannot be "in love" with it.
And because you put the needs of the other person above yours, you will always want what's best for them, even if that means going on separate ways. You will, of course, miss the other person, but without becoming obsessed with their presence.
Dolly Parton's "I Will Always Love You" (later reinterpreted by Whitney Houston) beautifully describes such a separation:
I hope life treats you kind,
And I hope you have all you've dreamed of.
And I wish you joy and happiness,
But above all this I wish you love.