The word "εθησαυρισατε" is a troubling word. It means to accumulate, store up or hoard riches. It shows up in James 5:3, as in "ethesaurisate en eschatais hemeraivs" or "You hoarded wealth for the last days."
James sees this as a problem.
Here're some possible explanations as to why:
1) You can't take it with you.
Wealth and the accumulation of your earthly life has a very limited and short-term value. How often do you see a hearse dragging a U-Haul?
2) You can't use it in the afterlife.
Maybe people thought they'd buy their way into heaven or a more comfortable afterlife. "Hey, St. Peter! I'll give you a beautiful, black BMW and a library of books if you hook me up with mansion near the river."
3) You can get distracted by it.
"All that is gold does not glitter" said Tolkien in reference to Aragorn. Our focus on the things we can store and carry may distract us from less tangible, more important things.
4) You can get tangled up with it.
The money in my 403(b) isn't available. Don't ask. Not even if you're doing something amazing for God or important for someone in need. These tangles on behalf of tomorrow keep us from being generous today and who knows if we'll feel like being generous tomorrow.
5) You might lose sight of your rank.
"Money can buy happiness" according to Daniel Tosh. "Money buys jet-skis." A large accumulation of money may make us happy, may make us feel comfortable, may make us feel secure, as if we are beyond life's fluctuations. A treasure trove may make us feel that we deserve special attention, like the frequent customers who justify their persnickity coffee order on the grounds that they come in every day (yes, I worked "grounds" into the coffee example on purpose). No amount of money can make you the Lord of the Sabbath.
Why do we hoard our money? Why do I put money in a 403(b)? What are we doing?