Because some people abuse what they know. Just like how some people abuse their power. They feel like they know or have or do better than someone else they try to make someone feel bad for not being doing or having what they have to make themselves feel and or look better about what they got. They feel good about themselves "knowing" they have the "truth"
It isn't right but just know that it isn't what or who they serve being the reason they try to force their religion. But because they don't know how to properly disciple. I for one am a Christian and I try to help people who need help. But if someone doesn't want to hear what I have to say or hear my advice in a situation then I stop and I'm fine with it :)
