But the question should not be why does He say this is our mission, but rather why does He have to tell us to do it? You would think that something so profound and life changing would not require being commanded to share it. So why did Jesus put so much emphasis on something that should come naturally to us?
I think it's because He knew it would be the hardest thing for Christians to do because of our human nature to avoid confrontation and vulnerability. We live in a society where people will refuse to have the simplest of all conversations when they have to confront or apologize to others. Like cowards we will justify the guilt and shame away. I am the best at this. I can come up with an excuse for everytime I feel led to do something out of my comfort zone.
Especially sharing my faith. They might ask me a question I don't know and I will look like an idiot. Everyone has their own hesitations, but it all stems from fear of rejection. The fix is in shifting our perspective. Take Peter and John for example. Following Jesus ascension, they faced the threat of death, but they said, "We cannot help speaking about what we have seen or heard." They didn't know the answer to every question, but they did know that lives had been changed. How can anyone refute what Christ has done in my life? And what's the worst that could really happen anyways?
Too many times we hit the "easy button" in our faith and become complacent in our own human nature. It's my prayer to seek out opportunities to be bold. Someone was bold enough to share it with me...