Like in any relationship, not necessarily romantic, you get into a more comfortable, less aroused state when you're with the other person. By arousal, I don't mean physical. The novelty wears off. You're more relaxed in general. Of course, plenty of interesting and worthwhile things happen when two people are that comfortable together. You can still make "sparks fly", whether with an old friend or with an old lover, it just requires a more conscious effort.