Because each religion think that their methods of worship and beliefs are the 'right' way to the path of heaven or reincarnation etc. They push religion on others to promote their religion and to gain more followers.
Some people are just trying to spread the word of their beliefs. I'm religious, and sometimes I mention that god can help you or me, or say I thank him and stuff like that. I don't say it all the time. i do know some people who push their religion on everyone they meet, so I understand what you are talking about. It's not bad for someone to be very religious, but they do need to respect other peoples beliefs just like they want you to respect theirs.
It is our responsibility as Christ followers to tell all non-believers about the Truth of God and of Christ and of the great Gift of Grace God gave humanity through Christs sacrifice. We are to TELL people not force It on people however. What many Christ followers/Christians fail to grasp it seems is that ALL we are supposed to do is love people and tell them about Christs love. It is not our responsibility to judge or to make others accept the Gift, just tell them about it. We will continue to do so because it is what God requires of us but hopefully new generations of Christ followers will do what they are supposed to do with more love and much less arrogance.
It is all that some people know how to communicate with. It has replaced ordinary common speech patterns with symbolism and hidden ways to guide your thoughts to agree with there agenda. They mean no harm they just don't understand that some think it's rude to presume in speech. The language of logic is seldom used in a world where it does not apply.
There is a story that goes like this - A fox got its tail cut while stealing a chicken. Sensing the horror of being criticized & mocked by his fellow foxes he devised a plan. He called all the foxes of that jungle and described to them how wise, good & smarter he felt without his tail! Then he insisted that all the foxes of the jungle must also have their tails cut to derive the same benefits as he did. Moral of the story being, that a fox without a tail will always insist other foxes to sacrifice their tails. Same with the religionists.