Jehovah's Witnesses believe they are the only true Christian religion on the planet. Because their viewpoints are so different, they assume that they must be the only ones with the truth. Matthew 7:13-14
Because they sacrifice so much of their time to Jehovah's visible organization (the Watchtower), they assume they're more worthy of salvation than all other Christians.
Also they feel their beliefs are better because they have added rules and regulations for righteousness that go far beyond what Christ required. They do not consider the possibility that because their doctrines are so radically different from what Jesus taught, they might be the only false religion in the world and all other Christians have the truth!
They have definitely run ahead of the words Jesus spoke and here's what the Bible says, 'Anyone who runs ahead and does not continue in the teaching of Christ does not have God' 2 John 1: 9 - 11 NIV
The best way to understand Witnesses is to research good web sites. Focus on sites that expose information the organization hides from it's members. You need convincing factual evidence, that proves the Watchtower is not God's 'visible organization' or 'modern day representative'. Fortunately there is plenty of evidence right in the Watchtower itself! Since Jehovah's Witnesses do not trust information unless it comes from the Watchtower, it is best convert them by quoting passages directly from the organization's literature as much as possible.