There are a couple of difficult relationship areas for the ENFP. The first problem is that many ENFPs have a problem leaving bad relationships. They tend to internalize any problems and take them on their own shoulders, believing that the success or failure of the relationship is their own responsibility. As perfectionists, they don't like to admit defeat, and will stick with bad situations long after they should have left. When they do leave the relationship, they will believe that the failure was their fault, and that there was surely something they could have done to save the relationship.
On the entirely other end of the spectrum, many ENFPs have a difficult time staying focused and following things through to completion. If they have not focused on their ability to follow through, they may have problems staying in dedicated, monogamous relationships. They are so in tune with all of the exciting possibilities of what could be, that they will always fantasize about a greener pasture out there somewhere.
If they are not paired with a partner who enjoys new experiences, or who shares their idealistic enthusiasm, the ENFP may become bored. The ENFP who is bored and who is not focused will be very unhappy, and will eventually "leave" the relationship if the problem is not addressed.