the reasons people have for living include items in the list below but before this list, the word why can be dissected into two components: origin and purpose: what starts the process and what the end result is
the whole of creation is considered by religious people to be the work of a creator/god (s) this includes both the origin and the purpose so our lives, also, are neatly given their answer by religion in one fell swoop.
for non religious people the answer to why we live has two different components 1. the origin of our lives: how humanity got here 2. the purpose of our existence
for the first part, how we got here the answer is in science wallace, darwin and richard dawkins satisfy this curiosity
but secular people consider the purpose of existence as what they find or give for themselves
meaning purpose function future family friends finding, (discovering, exploring, in the physical or abstract world) first, ( being best, superior, excellent, dominant, a leader) financial, wealth comfort success victory over adversity control commitment peace altruism selflessness revenge satisfaction of curiosity avoidance of death
but all of these things only lead to one outcome which is happiness or contentment
so for the secular world our purpose in existence is to achieve happiness, contentment or whatever other word for positive emotions the reader chooses.