You didn't get it. The soldier throwing himself on a grenade is acting in his self interest; his self interest is protecting his comrades. Self interest is not necessarily at the expense of others.
suicide is the opposite of self-interest...or maybe the height of it depending upon your pov.
Self-interest is not self-preservation, and it's not "get mine at all costs". It's acting in the interest of what one values most. Value to humans is NOT solely financial, although in economic models it can be approximated financially when you;re working in aggregate most of the time.
Value to humans is based on many things including time, emotion, sentiment, trade-offs, ease of burden or stress, and of course price. They vary from person to person, and from time to time and situation to situation. I might value my cup of tea in the morning a lot more than you do (especially if you're a coffee drinker). I value it a lot more now that I'm in my 40s than when I was in my 20s and didn't so much need the pick-me-up each day. I value it more on work days when I need to get going in a short period of time than on weekends when I can take my time waking up.
A soldier who throws himself on a grenade might very well be acting in his own self-interest, because he values the sum of the lives that he is saving more than he values his own. It's not suicide, it's self-sacrifice.
Most of the time, people act in their own self-interest - they act in accordance with what they value most. Just because you can't read their minds, and don't know what those values are doesn't mean they don't exist, at least
most of the time. Some exceptions exist - they always do. Not everyone is always rational. Even accounting for the schizophrenics and the mentally disabled out there, some people sometimes just react without thinking and get it wrong - we have regrets after all. But often enough to be generalizable to the "model" it holds true.