Apple Inc (AAPL  ) shares are up nearly 40% for the year even though the company has had a couple of what Deepwater Asset Management's Gene Munster calls "sluggish quarters." A new investment theme has emerged, he says.

The Case For Cupertino: The investment theme for Apple stock changes every five to 10 years, Munster said. Recent price action shows that it's occurred again, he said Monday on CNBC's "Squawk Box."

"I think there's a bigger theme going on here and that is this installed active base. Two of the last three quarters, Apple has led their press release emphasizing this point, that this continues to grow," Munster said.

Investors are beginning to show signs that they are giving more weight to the installed active base, he added.

When someone buys an Apple product, they usually fall in love with the product and start exploring other Apple offerings, Munster said. That Apple user is also going to upgrade to newer versions of the products they buy down the road, he noted.

"The previous big theme was, of course, related to services. Now I think we're entering the theme related to the active installed base," Munster said.

He told CNBC that he put together a waterfall model to frame how it's going to play out in the future and found that Apple can continue to grow its active installed base even if device sales are down 15% to 20%, he said.

"I think this is going to be a reassuring factor for Apple investors and something that has been overlooked with all of the noise in earnings," Munster said.

"Based on Apple standards, these have been relatively sluggish quarters ... I think the stock performance is evidence that there is something bigger going on."

AAPL Price Action: Apple shares were down 0.39% at $178.15 Tuesday morning, according to Benzinga Pro.