With any relationship, the longer and better you know someone, the more comfortable you are with them. While this is a good thing, it can also backfire after awhile.
When you are in a romantic relationship, the last thing you want is for your partner to get bored with you. It happens, and it happens often. If you aren't able to keep their interest and help keep that spark alive, well then you will probably be heading for splitsville soon. (This goes for both parties of the relationship.)
The best way to keep a relationship fresh? Do new things together. Take a cooking class. Try a restaurant neither of you have been to before. Have him or her teach you something. The important thing here is to try. Move out of your comfort zone, and before you know it, the two of you have something brand new to talk about. Don't take each other for granted and just assume the other one will pick up the slack. Sometimes you have to be the one to initiate change.