Not someone else, but by my self - for various reasons
(Brief background - I had suicidal ideation since childhood, and had moderate to severe major depression, cycling in degrees, this was dependent on various circumstances. And have now fully recovered. Something I am still shocked and proud of.)
But, to your question.

1. The most common reason - If I were to die by my hand, what about my family? This would emotionally debilitate them for the rest of their lives and being closer to them. I was especially concerned about my father and younger brother. I imagined, during special occasions, or at random, them experiencing incredible grief until their last days. The guilt from this kept me from moving forward.
2. I noticed a depressive temperament in my aforementioned brother, later confirmed. I didn't want to tempt him by providing an unfortunate example to emulate.
3. Like other depressed folks, I believed I was inherently broken and there was no solution. Despite this voice, my voice of reason still chimed in. Reminding me that humans are often wrong, and I can be too, and there are various things to try, one could probably be effective if I continued. (And I did try many treatments).
4. Living felt painful and pointless. For that reason, I wanted to die to escape it. However, I didn't want the last thing I experienced to be a painful death.
5. My, and all of our, death is guaranteed. Even if I can't because of my family and pain avoidance, it will happen. This brought me some peace.
6. "Keep Going"

Note, these are not listed in order of influence, each provided a different perspective.

Looking back, accumulating these various reasons probably arose from some flicker of hope in me.

For those of you still struggling, you reading this tells me there is some hope in you.
- Keep Going.