What a great question! You do not say if you are a Christian or not so I am going to answer it for the believer and nonbeliever. I am also giving my opinion according to how I have studied and allowed the Holy Spirit to reveal truth to me.  

In this life on earth I see many guarantees. Eternal life is guaranteed for all humans. We shall all live forever somewhere. Forever with the Lord if we believe Jesus Christ died on the cross for our sins and we put our trust in Him as our Savior. Or forever in hell if we have refused to believe that we need a Savior!  

Just believing in God is not enough to guarantee forever with the Lord. We must be born again, believing as Jesus says in John 14:6: I am the way, the truth, and the life. No one comes to the Father except through Me. 

This being said, when we are true Bible-believing Christians, we know we live in a fallen world and that good and bad things happen to everyone. But we can stand on 1 Peter 3:18: For Christ also suffered once for sins, the just for the unjust, that He might bring us to God, being put to death in the flesh but made alive by the Spirit. Knowing that I am born of God is true happiness on this earth. Because no matter what comes my way, my happy place is in Jesus Christ and His promises! 

The word happy in the Hebrew means to rejoice, be glad, or brighten up! In the Greek it means blessed, fortunate, well off. The English says it is a feeling of pleasure when something nice has happened or a feeling of satisfaction in life. With all of these definitions in place, believers have the Word of God to help them when unhappy things come their way. When we read and obey what the Bible says, His words will get us beyond where we are emotionally to where we are supposed to be emotionally.

Emotions come and go! Situations come and go! People come and go! But our Lord is and will always be with us wanting us to be joyful and at peace no matter what good or bad situation we are facing. The apostle Paul knew who he was in Christ and he knew his journey on earth. He was on trial for his very life when he spoke how we can be happy on this earth no matter what…. I think myself happy Acts 26:2. Paul’s whole life after he became a believer was to please the Lord. When we want to please the Lord, this will bring peace, no matter our situation! 

Your kingdom come. Your will be done, on earth as it is in heaven. Matthew 6:10