Why do some people insist that we should all love each other?

I am not old. I am not a kid any more either.
I am not a genius, nor am I stupid. I have been around the block a few times, had some great times, seen and done some shit that I am happy to forget.
So I figure I am pretty average at that point.

I hear a recurring message that we should love one another.

There are some people that I truly love, in fact a lot of them.
There are more that I am truly ambivalent about.
And there are a few that I justifiably despise. I find myself occasionally hoping that they will throw a punch in my direction so I can legally exorcise my negative passions.

It is not reasonable to expect me to love everybody. So what is that nonsense supposed to mean to me?