We are attracted to those that we find it satisfying and gratifying to be with. If a relationship gives us more reward and pleasure than cost and pain, we will like that relationship and wish it to continue. Thus, even after a relationship ends, we may find ourselves drawn to people that remind us of the former person.
This can help explain why no love can feel quite the same as that "first". These "firsts" can generate sensations so new and unfamiliar that the experience feels almost unreal. Besides emotional engagement, these experiences also have a heavy dose of novelty.