Because they're greed and they feels like their gf/bf is just their lover and they feels like they need someone else other than their lover in their life and they keep on giving false hope eventhough they dont really have feelings for you but smtimes they dont really know that the little things they did for someone else may make someone fall in love with them i dont really know what kind of person they really are but if they're really make you fall for them and keep on entertain you like you're his/her lover i'b better say these people are not worth to keep you're just hurting yourself if you keep on having frrlings towards them i know it might be hard but move on is the best way