Purpose and happiness do not have to belong together. One could create a purpose in exclusion of happiness, for example, your parents could say"your purpose in school is to get straight A's and get into UT regardless if the process is or produces joy or not". But your parents could create a purpose that doesn't exclude your happiness, that is they could say "get straight A's, get into UT, and you must be happy in the process and it also must produce more joy".
According to Q1 of the Westminster Shorter Catechism, God's ultimate purpose for humans includes our happiness, that is "the chief end of man (the main purpose of man)" is to glorify God (make Him look valuable) AND to enjoy him for ever. So God has created a universe that commands you to glorify Him AND to find your HAPPINESS in HIM (Piper would say this is one and the same, that is glorifying God IS enjoying God (I agree)).
Q2 of the Shorter Catechism, tells us to the rule or authority on how we may glorify and enjoy God, that is through the authority of the Bible. So mediating and reading on the Bible isn't something we do in exclusion of our happiness, but the instrument that teaches us how to glorify and enjoy God.
Q. 1. What is the chief end of man? A. Man's chief end is to glorify God, [a] and to enjoy him for ever. [b]
Ps. 86:9; Isa. 60:21; Rom. 11:36; I Cor. 6:20; 10:31; Rev. 4:11
Ps. 16:5-11; 144:15; Isa. 12:2; Luke 2:10; Phil. 4:4; Rev. 21:3-4
Q. 2. What rule hath God given to direct us how we may glorify and enjoy him?A. The Word of God, which is contained in the Scriptures of the Old and New Testaments, [a] is the only rule to direct us how we may glorify and enjoy him. [b] [Flavel]
Matt. 19:4-5 with Gen. 2:24; Luke 24:27, 44; I Cor. 2:13; 14:37; II Pet.1:20-21; 3:2, 15-16
Deut. 4:2; Ps. 19:7-11; Isa. 18:20; John 15:11; 20:30-31; Acts 17:11; II Tim. 3:15-17; I John 1:4