Sometimes love has a way of sneaking up on you. It's not uncommon for someone to be in a "comfortable relationship" doing all of the friendship/companion things, having sex, traveling together and so forth....etc
One day this person you "do not love" meets someone who flirts with them, makes them feel desired, and stirs romantic feelings within them. He or she tells you they believe they may have found "the one". So they pack up their bags and move out. Weeks go by and you come to the realization that you actually did love them! The whole time you were together you down played your emotions, took them for granted, or assumed you could find a replacement in a heartbeat. Now you find yourself wondering "How do I get my ex back!"
My point is maybe this relationship is not as "benign" as you think it is. You may care more than you want to admit to them or yourself. Some people seek to avoid becoming "emotionally invested" in their relationship in hopes of "protecting" themselves. However the more time you spend with someone the more you come to rely on them. Love is expressed and felt differently for everyone. There may not be fireworks going off in your heart or music playing whenever you kiss each other...etc But if the thought of you not being in each other's life disturbs you then your feelings run deeper than you suspected. You don't want to find out you love someone after they are gone.