Co-dependency. The therapist in me would say "a co-dependent relationship is defined as a relationship where much of the love and intimacy in the relationship is experienced in the context of one person’s distress and the other’s rescuing or enabling". In simpler terms, you felt most loved when doing something for your partner that may or may not have been the best thing for them. I.e. paying for his/her bills and your own, when you both make the same amount of money; taking/defending your partner's side without questioning/understanding the whole picture. Although by themselves, these situations are not deal breakers and do not automatically signify that you are in a bad or unhealthy relationship. However, if this becomes a pattern where you feel most close and bonded to your partner when coming to their rescue or defense, then you are likely engaging in a co-dependent relationship. Even though these relationships can last and being co-dependent doesn't necessarily guarantee a break up, it is an underlying reason for breaking up if one person no longer needs rescuing or the other person gets worn out from doing the rescuing.
Denial. When there is an inconsistency between action and words, I would recommend to believe the actions over the words. Especially in a relationship. Ever heard of lip service? Most people know the right things to say and usually will say the "right" thing to make a relationship work. What makes this bad for a relationship is that you want to believe in what your partner tells you, so you are more likely to overlook the inconsistency in their words and actions. Sometimes it takes stepping outside of yourself to really see the clarity of your partner's behaviors. I worked with a couple once in which there were communication problems. Although this couple loved each other, they could not see just how detrimental their communication problems were to their relationship. It wasn't until the wife saw how her husband talked to others (in the same angry manner towards her) that she realized the emotional damage she experienced in their marriage.
Unrealistic expectations. This is also what I like to call "fantasy" or "romanticized" relationships. Although I won't list all of the possible unrealistic expectations, it is important to note that most beliefs that are finite or absolute (ie. my partner should share all of the same beliefs as me or my partner should always know how to console me/make me happy) usually fall under "unrealistic". I'm not bashing on anyone for wanting certain qualities in a partner, but my point is something totally different from that. Wanting certain qualities and expecting these qualities to be fulfilled in full are two separate things. I am referring to the latter here. The effect that "holding onto unrealistic expectations" has on a relationship is strong. It shapes your feelings towards your partner and contributes to emotional disconnection when the expectations are not met.
Low self-esteem/self worth. This is often at the core of many broken relationships and can be related to any of the above mentioned reasons for a break up. This impacts a relationship because subconsciously you will seek your partner to improve your esteem/worthiness, only to feel disappointed when your partner disapproves or disagrees with you. Not surprisingly, it's very difficult for anyone to admit to themselves let alone to their partner, but nevertheless it drives behavior. It may have driven you to cheat on your partner, be with the prettiest/hottest person in the room or driven you to lie about your income/insecurities/faults. Unfortunatelly, these behaviors do not make you love yourself any more.
Sometimes the end of a relationship can be the best thing for you, especially if any of the above reasons were present in your relationship. Hopefully, reflecting and identifying what went wrong this time around is a good way to move on towards a healthier relationship next time.