The song is just more heartbreaking/heartwarming as you analyze it. The Pikmin are doing this all because they care so much about Olimar and want to help him. So badly that they'll put themselves into danger, fully knowing that some of them will die, just to help Olimar. And they want nothing in return, not even Olimar's love. And they even strive to try harder. The Pikmin are the most selfless creatures ever, and they don't even ask for recognition.
The counterpart to the above song, Namida Ga Afureta, is just as heartbreaking. Translated to 'Tears Overflowing', the song is in Olimar's point of view, who regretted that he could never truly express his gratitude and love to the Pikmin he owed his rescue to at the end of the first game.
Of course, given that in the sequel he really had all the time in the world, this leads to FridgeHeartwarming for Pikmin 2.
If you let yourself get attached to them, a single Pikmin death can be this.
When a Pikmin dies its soul floats away with a depressing half moan half wail.
Did you forget any Pikmin out in the field at the end of the day? If so, you're treated to cutscene of the left behind Pikmin being chased and eaten by the area's predators. Or even more heartbreaking, the remaining Pikmin desperately trying to make it back to the ship as it takes off without them... and it becomes worse in the third game, where they are given unique cries in panic when this happens.
Bulbmin. Sure, they're part nightmare fuel, but think about it. You kill their leader and then they turn to you. You use them along with your trusted allies, and they get you out of tight spots...then, you abandon them when you leave, and they just look up at you and frantically swarm the geyser...what happens to them?
Pikmin Extinction. This occurs when you lose every single Pikmin you have and having none left in the Onion. Hundreds of little plant creatures... and you're responsible for them all dying. Feel bad yet? Olimar certainly does:
"I wonder if I shall ever be able to escape from this world. How much suffering must I endure before I can finally see my family back home again? Still.... When my heart grows too heavy, I take comfort in my efforts. I will get home...or I will expire trying."
"I suppose I have done well to get by without much motivation. I have made it this far without any hope."
If you have less than 25 parts by day 30, Olimar manages one sentence:
"I am so very tired."
If you lose a large amount of Pikmin in 3, Captain Charlie may make a diary where he holds a moment of silence, then says that watching a soldier perish is something that you'll never get used to, but you have to carry on anyway.
The Golden Ending of the first Pikmin can be pretty emotional, watching Olimar bid the Pikmin farewell as they look on sadly.
Think Olimar gaining control of the Pikmin ruined their lives? This fan video might make you think again. It begins with a large crowd of Pikmin together simply relaxing, messing around, or even falling in love. Then a mass of Bulborbs attacks and drives them away. It only goes downhill from there. Sheargrubs eat some, a group of four is cut in half when a Snitchbug carries two away, and it's cut down to only a pair of Pikmin. Music begins playing as they finally find nectar, implying that things might start going another direction... before a Burrowing Snagret eats one of them. The last Pikmin runs in panic, finally resting to look up at the night sky all alone and think back about all of the previous trageties. Wait, night? When the creatures get more active? It finds itself surrounded by a number of predators, before the camera zooms out to reveal a certain crashed ship. It's very slightly implied that the race somehow survived to make the Onions Olimar would later find and use to help rebuild them, but still, damn.
After spending the whole second game with the Jerk with a Heart of Gold Hocotate ship, it was pretty jarring to see it broken and silent in the ground in Pikmin 3. One can only hope they eventually dug it out and repaired it.