Why we make effort to keep friends?

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I sometimes wonder why friends keep in touch with each other? Why make all that effort when one has so much to do? We need them for the following maybe?

to pass our free time?
for company?
for networking?
for business information?
to use them?
borrow stuff or money when in need?
tell people you are well-connected?
use your friend's connections sometimes?
So that they can tell something good about you to others?
Job referrals?
sport partners?
rich family?
swear together?
join your club?
convert them to your religion?
see your friends wife/ sisters?
stock/horse tips?


I want to hear your thinking straight from the heart?