"Pioneers! O Pioneers!" has about eleven billion exclamation points. We're not exaggerating (okay, we are). Seriously, when you have that many exclamation points, you know that you're dealing with someome who is excited about something, someone who admires something, someone who really really likes something. For our speaker, that something is pioneering. For him, the work of pioneers is among the most important work there is. They perform the work of creating and expanding a nation, the work of pushing out into the unknown. And our speaker he feels his own work is to praise and admire them, to urge them on, to mourn their losses and rejoice in their triumphs.
Our speaker admires pioneers for their bravery.
These pioneers are admirable because they never quit, and they're never satisfied.