"We may never fully understand where these men and women find the courage and strength to do what they did," Obama told the more than 2,000 people at a Missouri Southern University memorial service Sunday, a day before the first funerals for the 142 people confirmed killed in the May 22 tornado were to begin.
"What we do know is that in a split-second moment where there's little time for internal reflection or debate, the actions of these individuals were driven by love -- love for a family member, love for a friend or just love for a fellow human being," Obama said.
These people stayed in the thick of the storm to save others, with no thought of what it could mean to their own lives, Obama said.
The United States will honor the service these people gave to others and to life itself by helping Joplin rebuild, Obama said.
"The cameras may leave. The spotlight may shift. But we will be with you every step of the way until Joplin is restored and this community is back on its feet. We're not going anywhere," Obama said to applause.
House Majority Leader Eric Cantor, R-Va., said Sunday that while the government would "find the money" for emergency assistance to Joplin, the aid would need to be offset by federal cuts elsewhere.
Speaking on the CBS News program "Face the Nation," he likened the federal government to a family on a tight budget.
"When a family is struck with tragedy, like the family of Joplin ... let's say if they had $10,000 set aside to do something else with, to buy a new car ... and then they were struck with a sick member of the family or something, and needed to take that money to apply it to that, that's what they would do, because families don't have unlimited money," Cantor said.
"Neither does the federal government" have unlimited money, he said.
A key House committee approved $1 billion in funding for the Federal Emergency Management Agency Tuesday.
FEMA has about $2.4 billion remaining in its disaster-relief account, but it's too early to know the final costs stemming from more than 1,300 tornadoes recorded so far this year and historic flooding in the Mississippi River basin, an agency official told The Wall Street Journal.
The estimated insured losses from Joplin's tornado disaster could reach $3 billion, catastrophe modeler EQECAT Inc. estimated. This would make the Joplin multiple-vortex tornado the costliest tornado in U.S. history.