Whoever loves his life loses it, and whoever hates his life in this world will keep it for eternal life. (John 12:25, ESV)
The Pharisees loved their life and so plotted to kill the Author of Life, signing their own death warrant. In contrast, Mary didn't care about her "life" but humiliated herself at Jesus feet, using her most precious possession to anoint him.
If we are building our lives on anything but Jesus, then Jesus is a threat to that life. But don't stop there. He's a threat to that "life" because what he's got for us is so much better.
Following Jesus might make you seem naive or bigotted or unpopular. But living for that reputation is like slavery. Jesus sets you free from that because there is no reputation with him. He knows exactly what you are really like and, despite that, he loves you. Your reputation might die but you’ve found real life.
Jesus may threaten your relationships because he challenges you to really love that person rather than acting in a way that causes them to continue loving you. Losing your need for love from other people because you’ve found the love you need in Jesus sets you free to really love other people without needing anything in return. The person who desperately needs love and affirmation from others has died but you’ve found real life.
Jesus threatens your finances because he says you should hold your money loosely and give generously. That’s threatening to both the person with money and without it. But when you see everything as a gift from God, you’re free to enjoy the gifts he’s given without looking for the next newest thing to buy. When you see that God loves you and is more committed to your good than you are, you are set free from the anxiety and envy that money can produce. The human money exchanger has died but you've found real life.
Jesus threatens your time because he says it’s his! But when you see the cross and see a God who loves you that much, you know that whatever plans he has for your time are better than the plans we had. The schedule may have died but you've found real life.
Jesus threatens your desire to be in control of your life and your plans. But when you realise that he really is in control, and he loves you - as demonstrated at the cross - then the control freak dies but you’ve found real life.
Jesus threatens your mask of strength/wisdom/independance because if what Jesus says is true then you are weak and foolish and totally dependent on him. But when you acknowledge your weakness and foolishness you see his strength and wisdom more clearly. The strong, wise, independent persona may have died but youve found real life.