The man who loves his life will lose it, while the man who hates his life in this world will keep it for eternal life.
We often think of someone who loves life as a happy-go-lucky kind of person. They are carefree and tend to take life as it comes and not worry about the bumps along the road. As Jesus has taught us not to worry, this kind of love of life should be encouraged.
When Jesus refers to a love of life in this passage in John, He is painting a love of life in a negative light. The person who loves life in this regard is someone who is attached to all that goes on in life and is unwilling to give up the pleasures of life.
People who are attached to the things of this world are unwilling to sacrifice them for the good of others or even the good of themselves. In contrast to this person is the one who hates his life in this world.
This obviously isn’t talking about a literal hate or a reference to someone who is suicidal. Instead, it is referring to someone who is willing to sacrifice and has not placed the cares of this world above all other things.
Like many things, this comes down to priorities. A person who loves his life is going to be focused on the here and now and their priorities will be on enjoying life. The person who hates their life will realize that they are looking for eternity. Their priorities will be focused on the things of God and not the things of this world.