Here are some possible answers to the question of what makes life worth living: (1) nothing; (2) religion; (3) happiness; (4) love, work, and play. Evidence from psychology and neuroscience supports the fourth answer.
(1) Nothing. A few despondent philosophers such as Nietzsche, Schopenhauer, and David Benatar have cast doubts on whether life has any intrinsic meaning, and some people are driven to suicide by depression and negative events in their lives. But most people, fortunately, are able to find lots of reason to value their lives, and in surveys most people report themselves as pretty happy. So nihilism is not a plausible position.
(2) Religion. Surveys also indicate that many people report that religion and spirituality are major sources of meaning in their lives. Unfortunately, however, these sources are bogus if there is no evidence to support claims for particular religious beliefs. Religious faith may be reassuring, but cannot objectively tell you whether you should adhere to Christianity, Islam, Hinduism, or some other religion. Faith cannot even tell you what version of Christianity (Catholic, Baptist, Morman, etc.) or Islam (Shia or Sunni) you ought to adopt. Hence religion and vague spiritual ideas like "everything happens for a reason" cannot provide a sound basis for living.
(3) Happiness. Psychological research has identified many ways in which people can increase the happiness in their lives, as in Sonja Lyubomirsky's fine book, The How of Happiness. But happiness is usually the result of having a meaningful life, not what makes life worth living in itself. There are people whose lives are meaningful even though they may not be very happy, for example when struggling with a challenging job while raising a special needs child. On the other hand, happiness can be cheaply achieved by slacker serenity, a mindless bliss resulting from having minimal goals, access to drugs, or unlimited time for meditation. You can have happiness without much meaning, and meaning without much happiness; so happiness is not the meaning of life.
(4) Love, work, and play. In my new book, The Brain and the Meaning of Life, I argue that these three activities make life worth living. Love includes friendships and family relationships as well as romantic ones. Work includes diverse productive activities such as community volunteering in addition to wage slavery. Play includes all forms of entertainment such as reading and watching movies, not just games. Surveys and other psychological studies indicate that love, work, and play do indeed enable people to have lives they value. Neuroscience provides a deeper understanding of how brain processes generate needs for relatedness, autonomy, and competence that can be satisfied by the successful pursuit of love, work, and play. Such satisfaction yields happiness, but even the pursuit is enough to give life meaning.
Blog post to come: 36 arguments for the existence of God.