Why would anyone want to do that? Jesus wasn't talking about reasons why someone would do that, but the potential of what faith can do. Mountains don't have ears, so they can't hear you speaking. Even if they did, they couldn't understand what you are saying because they don't know languages. So then, who is listening to you speak to the mountain? What is making the mountain move? God is the one who hears your words, and He is the only who can make the mountain move. He is responding to your words spoken by faith.
Why didn't Jesus just demonstrate it for His disciples? He could have spoken to the mountain, pulled it up into the air, floated it over Israel and dropped it into the sea, and then said, "Did you guys see what I just did? That's what possible with faith!" The reason He didn't do it was because He had already demonstrated the power of the spoken word just before this in Mark 11:13-14,20-21 when He cursed the fig tree. After the disciples saw that the fig tree withered, Jesus told them, "What works for fig trees also works for mountains."
Are you speaking words of faith of what God will do, or words of unbelief? Proverbs 16:23 says, "The heart of the wise teaches his mouth." Rather than complaining about your problem (the mountain you face), teach your mouth to speak positively about what God will do to fix it.
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