I mean, there are plenty of people who tell their personal experience of why Mormonism is true or about how they saw aliens in a space ship, but that doesn't make me believe them. You have to be able to provide evidence, not just that Christianity works for you, but that it is actually true. Otherwise people are going to tell you (or think in their minds, even if they don't say it) that they're glad you're so happy, but they just don't think it's right for them.
The other problem that happens when you only tell your personal testimony is that you risk teaching people that Christianity is just an opinion - something that works for you. A lot of people think choosing a religion is a lot like choosing a favorite ice cream flavor. Everybody has a little bit different taste, so just pick the one you like. When you only give your personal experience, you are telling them that you like Christianity and maybe they should try it, but you aren't telling them why it's the ONLY way. It's of no more importance to them than you telling them you like one brand of something better than another and suggesting they try your brand. That's the way people see it when you only give subjective, personal experience - as a statement of your preference and maybe a recommendation, but not as objective truth.
But when you give that impression - that Christianity is just your favorite flavor of religion - you aren't actually sharing the gospel. The gospel is radical and exclusive. Christianity claims to be the ONLY way, not just one way of many. And that claim to be objective truth requires evidence. God doesn't want people to just "try" Christianity - going to church, being a good person, saying prayers. He wants them to believe it to be actually true. He wants them to believe that Jesus is the only way to be saved from the sin that is killing them spiritually and to place their trust in Christ alone for salvation. That's the gospel. Anything less is selling Jesus short and doing no favors to those we are speaking to.
Of course, we don't have to share the entire gospel with every person in every conversation we have. Most of the time, we won't have the opportunity to tell the whole thing. But we do have a responsibility to do what we can to leave them with some evidence that makes them question some aspect of their false worldview or have a better understanding of the Christian worldview. We have a responsibility to speak of Christianity as objective and knowable truth, not just our subjective preference, whether our conversation is long or short.
There is also a proper time to share our personal testimony. That should certainly be a part of what we do as Christians. However, it should not be the only thing we do or even the main thing we do. There is much more to the gospel than our personal experience of it. And Christianity is much more than simply what works for us. It is truth, not mere preference.
