New Answer: I think it is
Water. Again, water is existent in almost everywhere (though harder to find in areas like the sahara - unless your sweating). Water is essential for life to exist. In addition, two hydrogen atoms must combine to make a water molecule, and when combined, it can form life. However, for some living species water can be a curse (or feared) or mixed with substances cab make poison. Two water molecules alsi result in six total elements). But in the end water is essential to living.
Old Answer (Disproved by OP)
It is
Life. You can see evidence of life everywhere, and without life - well you don't exist. In order to have more life there has to be two living same species (except asexual reproduction). When we come together, etc. self explanatory. For some people life is a curse, or they don't like their life, but by living, life is helping you.