People are looking for what they need and want from a partner, and two people might not see the same things in the person they are dating. One of them might see the perfect partner who can make their life complete, yet they may not be as satisfying to the other person. Their need for someone to make their life better could be seen as a burden, and their partner might have a bit of hesitation to commit to a relationship with them. The two of them will not find happiness until they solve their issues, but they must first identify and discuss them.
There might be one or two perfect people in the world, but they are probably in hiding. Compromise is often a big component of any relationship, so no partner will be perfect when the relationship begins. Making a commitment to another person for the long term means defining strengths and weaknesses and deciding if they are acceptable. If one person believes their partner is the one who is in charge of doing everything to make the relationship work, their lack of responsibility could have them out of a relationship without trying.
Failure to be responsible and shoulder a share of the work that goes into building a life together is often what causes one person to leave another. Many people are willing to work for a good relationship, but they are seldom willing to do all the work. Making another person happy all the time is more than should be expected, so they should leave if that is what their partner is seeking.
Working out issues like these can be difficult, and a basic disagreement on the reason for being with someone often results in a breakup. For those who had a partner less than responsible, walking away without a commitment is often their best course of action.