Yes, you are correct (in my opinion). Happiness and sadness (and every-thing in fact) all come from within. It is vital to re-member that we are not these bodies. These bodies ae just vehicle for the One True Self. If we are able to remember this (both in times of happiness and sadness) and feel all the feelings (the bodily sensations) without judgement or blame (to blame is to be lame) then we will experience more and more joy. Joy apparently is the ability to be content with all the feelings, feeling them all.
As Kahli Gibran said (paraphrasing), sorrow is carving out a space in the being so it can contain more joy.
Sat Nam
Atma