The purpose of man is to know and love and serve him
To Know God: Man is made to understand and learn about God, which is essential before one can truly love Him. Knowledge of God is acquired through the teachings of the Church and personal study.
To Love God: As one comes to know God, the natural response is to love Him. The more one understands God's goodness and beauty, the more one is inclined to love Him.
To Serve God: The expression of love for God is through service and obedience to His will. This includes following His commandments and living a life that reflects His teachings.
Eternal Happiness: The ultimate goal of knowing, loving, and serving God is to attain eternal happiness with Him in heaven after death.
In summary, man was created to fulfill these purposes, which are interconnected and lead to the ultimate end of glorifying God.
SOURCES:
LESSON FIRST: On the End of Man
The end of a thing is the purpose for which it was made. The end of a watch is to keep time. The end of a pen is to write, etc. A thing is good only in proportion to the way it fulfills the end for which it was made. A watch may be very beautifully made, a very rare ornament, but, if it will not keep time, it is useless as a watch. The same may be said of the pen, or of anything else. Now, for what purpose was man made? If we discover that, we know his end. When we look around us in the world, we see a purpose or end for everything. We see that the soil is made for the plants and trees to grow in; because if there was no need of things growing, it would be better to have a nice clean solid rock to walk upon, and then we would be spared the trouble of making roads and paving streets. But things must grow, and so we must have soil. Again, the vegetables and plants are made for animals to feed upon; while the animals themselves are made for man, that they may help him in his work or serve him for food. Thus it is evident everything in the world was made to serve something else.
What then was man made for? Was it for anything in the world? We see that all classes of beings are created for something higher than themselves. Thus plants are higher than soil, because they have life and soil has not. Animals are higher than plants, because they not only have life, but they can feel and plants cannot. Man is higher than animals, because he not only has life and can feel, but he has also reason and intelligence, and can understand, while animals cannot. Therefore, we must look for something higher than man himself; but there is nothing higher than man in this world, and so we must look beyond it to find that for which he was made. And looking beyond it and considering all things, we find that he was made for God---to know him, to love him, and to serve him both in this world and in the next. Again, we read in the Bible that at the creation of the world all things were made before man, and that he was created last. Therefore, if all these things could exist without man, we cannot say he was made for them. The world existed before him and can exist after him. The world goes along without any particular man, and the same may be said of all men.
Neither was man made to stay here awhile to become rich, or learned, or powerful, because all do not become rich---some are very poor; all are not learned---some are very ignorant; all are not powerful---some are slaves. But since all men are alike and equal in this, that they have all bodies formed in the same way, and all souls that are immortal, they should all be made for the same end. For example, you could not make a pen like a watch if you want it to write. Although pens differ in size, shape, etc., they have all one general form which is essential to them. So, although men differ in many things, they are all alike in the essential thing, namely, that they are composed of body and soul, and made to the image and likeness of God. Hence, as pens are made only to write with, so all men must have only one and the same end, namely, to serve God.
The Creation of Man; Original Sin
Why did God create man?
God created man to know him, to love him, and to serve him, so that after death man might, by possession of God in the beatific vision, be happy with him forever in paradise.
God created man to know Him, love Him, and serve Him in this life, and so be happy with Him forever in heaven.
God made me to know him, love him and serve him in this world, and to be happy with him forever in the next.
Here we are face-to-face with the greatest problem of our existence---viz., why we are here. God never does anything, even the most minute thing, without having some end in view; and surely when he created man, the noblest work of his hands, he must have had some very special object in creating him. The catechism tells us that he made man for himself.
First, to know him. Now we can, and do, know a great deal about God, even though we have never seen him: 1) from the old testament, where God revealed himself to the patriarchs and prophets; 2) from the new testament, where God has made known to us much about himself and the next world through Jesus Christ, his only Son.
Secondly, we are made to love God. The more we know about him the more we see his infinite beauty and goodness, and thus we are induced to love him; for even in this world, we love what is beautiful and good.
Lastly, we are made to serve God. The way we show our love for God is by serving him, by doing his will. He himself has said, "If you love me, keep my commandments." The reward of all this---viz., of trying to know all we can about God, of loving and of serving him---is to be the possession of heaven, that happy place to which everyone is anxious to go. God has promised this, so we cannot doubt it.
So you understand now why you were made, why you are here: not to make a great name for yourself, not to get rich, not to enjoy yourself as you like, but simply to serve the God who made you. You are one of his creatures; you depend entirely upon him for everything. As your Creator, God has his rights over you, and the greatest of these rights is that you should know and love and serve him.