I think that it's easy to get used to be with good person, but after some time, you forget how it was without her. Than you meet someone new and have that feeling again, the same feeling like you had with her before. But it's not real, because that new person don't know you that well and she didn't live with you that much like your partner. It's important to be honest to yourself and know what do you want from the life. If it's onetime pleasure, or life with person which you love.