Our basic instincts are reproduction and survival.

Through the advancement of society and civilisation we have created meaning. These, in my opinion, are purely emotional and not physical like our instincts.

to acheive happiness

to eat shit and die

This

do 2 chicks at the same time ...and of course frag some noobs

death

This question is so hard to answer, because the answer is different for everyone obviously. Mainly it is different for everyone because everyone can determine there own purpose of life. So to put it the most simple way possible act1, the purpose of life is to do whatever the fuck you want to do. The purpose life is a choice that only you can make.
Once you set a goal in life, or a purpose, and accomplish it, you will find yourself much happier. This is because, if you are a male, whenever you accomplish something more testosterone is released into your body. This is because in a male body, testosterone lowers there stress level. Although if you are a female, oxytocin lowers your stress level. Oxytocin, in a female body, is produce with the being in relationships.
So overall the purpose of life is to set goals for yourself, and obviously accomplish them, but what goals you set is your choice.

I'd go with Carl Marks' saying about the 3 purposes of life:

1. Emotion
2. Survival

but the true meaning to life is, sadly,

3. Death

if you want a more expansive explanation, pm me.

why do we have to be here for a specific reason?

we are here because our fathers semen fertilized our mothers egg

just like my dog is here for the same reason

exactly, why do people need a purpose in life that's some sort of law set in stone?
why is it so mind boggling that one's life is simply coincidence and there is no fate or destiny to be fulfilled?
why not just live? or if one is not happy with living, why not simply die?

these are the questions that need answering