Why does my boyfriend want to live like we are married but don't want to get married?

Marriage is intended to be a lifelong commitment, until death do us part as the wedding vows say (although a large percentage of marriages do end in divorce) so it is a very serious decision to make, about how you want to spend the rest of your life. So your boyfriend may just not be sure that you are the right person to marry or that he is ready for marriage. Or, he may just prefer to live as a bachelor, and never be burdened by the responsibilities of having a family.