It might seem counterintuitive, but the person who is least responsible will often be the one to promise to do something. Their lack of maturity often allows them the ability to make promises they will never keep, and their partner is the usually the one left to make good on their guarantees. This is a failure to perform, and it should be a warning sign for those who are thinking about committing to a long term relationship. People who do not carry out their own promises are unlikely to be part of a stable relationship, and they will depend upon their partner to do all the heavy lifting.
Emotional attachments are not always rational, but partners who leave a string of broken promises behind them can help the person they are with to let rationality overcome their feelings. Someone who is constantly expected to fulfill a promise made by their partner will find resentment is a large part of their feelings. Their love and affection can often be turned into anger as they feel they are being used, and their attraction and attachment might eventually diminish under the weight of the problem.