We must pay very careful attention to what Jesus is saying in this passage. He is not simply saying that lust is as bad as adultery. What he says is more subtle, and more devastating, than that.
“Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them” (Matthew 5:17). Why would Jesus say this, and what does it mean?
Which would you rather have: the world's approval now, or God's approval forever? Because you can't have both.
Christ's disciples are meek, merciful and makers of peace, and they've been promised an incredible future. This is the good life.