When I focus on my breath, it becomes shallow Sometimes when I'm meditating, I'll have really "good" breaths - the breath will be deep, very satisfying and relaxing. My diaphragm and chest really expand and it feels like my whole lung fills with air. The trouble is I can't consistently maintain those breaths.

When I do focus on my breath in meditation it becomes more and more shallow. Not a lot of expansion, not relaxing, and not much inhale. I'm not sure why that's happening.

I think I get my best breaths when refocus on my breathing after I have been preoccupied with other thoughts.

Why does focusing on my breath cause it to become shallow and how can I avoid it?

P.S. I'm not against the practice of meditation. It has helped me with attention and focusing. I've tried other techniques such as being aware of the present moment with better user experience.
Doesn’t matter if your breath is deep or shallow.  Just let it do what it wants to do.  If you force it, you are introducing volition.  All that’s going to do is lead to tension and doubt.  Just let go of your expectations of what meditation ought to be.  Meditation is what happens, not what we make happen.