the rationality of man

Economics assumes that man is a rational being.
The rationality one assumes is based on theory of the intelligent promotion of one's interest.
I mean, we as rational human being would choose choices that serve us more than that which could hurt or impair us.
So, we as humans are of the tendency to self serve our self interest.

The note to consider is that in the matter of self-serve for reasons of self-interest; one can adversely affect ones relations with others.
I mean, why would anyone want to associate with one who plainly only sees his or her own interest and no other.
In such a case our own interest cannot serve our self-interest.

Which could mean that we are not as rational as we make ourselves out to be.
Perhaps true, we are irrational beings.
Or perhaps we define rationality erroneously.