The short answer to this is to know God and enjoy him forever.
The reason this is the meaning of life is because God created us that way. If you have a look at Genesis chapter 1 and 2 you see that God creates a beautiful world and puts humanity in it to live and prosper and to have a relationship with him. This was the purpose or meaning of our creation. What that means is that we are most happy, fulfilled and complete when we are in relationship with God.
The problem we have, though, is that we have ignored God and don’t live in relationship with him. This ia what sin is. And so our lives are less than what they could be. we live outside in the cold when God wants us to come into his house and live with him!
Only when we turn back to God and stop ignoring him will we find true happiness and fulfilment. Have a look at God Makes Sense to find out more.