Epicurus is my favorite philosopher. Although he lived in ancient time, his ideas are still pretty much relevant and applicable to modern lifestyles.
For me, the core concept of his philosophy is that the purpose of life is for us to be happy while we are still alive. He argued that when we die, we do not exist. If we do not exist, we cannot feel. In other word, he did not believe in life after death. So, we should be focus solely on our present lives.
He also held that before good and evil, there were only pleasure and pain. We feel that something is good because it gives us pleasure in some way. So, our ultimate goal in life is actually to pursue pleasure (and avoid pain).
However, one shall not blindly go after pleasure, because many types of pleasure, for example, sex, drugs, unhealthy foods, etc. could bring us pain in the long run. Therefore, we have to choose pleasures wisely to maximize our happiness over the course of our lives. In fact, he recommended three primary sources of sustainable pleasures for us to rely on i.e. to live with freedom from fear and greed, to have good friendship and to live one’s life intellectually.
I love the philosophy of Epicurus so much because it does not make people feel guilty about themselves going after happiness. The first time I heard about his ideas, I felt so liberated. We can do anything as long as we know our limitations and consequences. His ideas are also rational given the fact that we still cannot prove if God exists. It just makes logical sense in its own rights and fits nicely with modern lifestyles.