Dortmund have little to play for in their remaining three Bundesliga games, after their 4-2 win at home to Mainz secured their spot in the group stages in Europe in 2014-15.
Jurgen Klopp's men can fall to third, but while that will not affect anything but their pride, Reus wants Dortmund to build some momentum in their remaining matches.
Reus added his side's bid for silverware this season in the DFB-Pokal final against Bayern Munich in May could be bolstered by some positive results.
"We've still got four games left, so it's important we don't let our performance levels drop and keep improving," Reus said.
"We also want to take the DFB Cup back to Dortmund, so we mustn't rest on our laurels. We have to keep going."
Nuri Sahin was at fault for Mainz's second goal, as his attempt to find Australian goalkeeper Mitch Langerak was intercepted by eventual brace scorer Shinji Okazaki.
"You see bad back-passes all over the world," he said.
"I've seen it a thousand times on TV and now I've experienced it. It's an experience I didn't necessarily want, but that's football."
Mainz hero Okazaki moved level with Shinji Kagawa for most goals in a Bundesliga season by a Japanese player on Saturday - 13 - and he said he wanted more in the run-in.
"It's a shame we lost the match despite scoring twice," he said.
"It's a bit of a dampener after equalling Shinji Kagawa's Bundesliga record with my 13th goal of the season.
"I'll try and score more in the last few games so we can win again and I can beat his record."
