One does not think "Ah, my eyes hurt. I think I'll blink to apply some moisture." No, blinking comes from within, unbidden, and without regard for your rational faculties and will.
Love is like an instinct. You are correct in stating that loving someone is not always rational. The human brain is sloppy. For example, we have some sort of process which helps us to recognize faces everywhere we go, but this results in seeing faces when there are none. Similarly, we have the instinct to love people, particularly our children becuase it's a survival advantage, and not surprisingly, sometimes our love is applied in the wrong places.
Basically, all you're saying is that humans do random things, sometimes stupidly so. This is not at all surprising, because, once again, we aren't perfect in any way.
That may not be what you're talking about, but there's essentially no difference between them. They're just different emotions that occur for different reasons.
In some cases, indeed, there is nothing rational about such acts--like in your example. However, there's nothing mysterious or romantic about what you're saying. In most cases, these acts of sacrifice ARE beneficial to society. In other words, altruism is a survival advantage to the species as a whole (an aspect of evolution that many people overlook). Yet, once again, the methods of the brain are sloppy, and our instincts will force us to apply such altruism to situations where they don't help anything--like your example.