Riddle: I can be this, and you can be this, and, yes, we can be this

I can be this, and you can be this.
And, yes, we can be this.
He can't be this and she can't be this.
And no, they can't be this.
Dogs can't be this, but cats can.
And a kitten can't be this, but a puppy can.
Givers can't be this, but beggars can.
And humility can't be this, but greed can.
Not even peace can be this, but fear can.

What am I?

Source: I don't know where I originally saw this as it was many years ago. But this might be the original source, posted in 2003.

• Today I learned that I've been rot13(glcvat) wrong all this time, I always rot13(cerff gur o xrl jvgu zl evtug unaq). – Davy M Jan 14 at 23:10
• Now I think about it, so do I! – Earlien Jan 14 at 23:17