Your bf/gf is a person you meet and love. Even though you two 'love' each other. You have extra stuff to worrie about. But with a friend you two are friends and don't have to worry about 'he looked at her the wrong way' or 'you don't live me anymore'. There is less to think about and put between the each other. But if you were in a relationship that a whole new level of topics to think about.
Loving them dosent mean you spend time with them. There are people here in the United states that love people in Africa, even though they never meet them, yet they still love them. Love can be a variety of things, mental love, emotional love, physical love. You can love a person in any way you want, and yet you may have never met the person before.
It's basically like theres a slight chance that you're gonna date a guy you had just met forever, but there's a higher chance that your best friend since like kindergarten will be your friend longer than the relationship that you have with this guy. And you have to think about the person who will be there for you and with you through anything, no matter the situation. Like if the guy got you pregnant, if he's an but hole, he'll leave you, but your best friend will be there for you and go through it with you. Though that was just an example.