Because he/she does not feel safe with you or does not trust you to be able to hear his or her truth without rejecting, judging, or punishing. People may also lie to themselves and other out of shame, fear, or guilt. Telling the truth can be a very vulnerable and scary thing.
The best you can do is to be curious why a person lies (and maybe they just have a different truth than you) and work on yourself to become more trustworthy by showing compassion. Of course, if people don’t change and abuse or harm you by lying, then you need to show healthy boundaries and end your relationship. People with a histrionic personality disorder lie to get attention and to make themselves bigger than they are. It comes from deep insecurity.