On Wednesday Fed Chair Janet Yellen reassured markets that the Fed is not yet ready to raise rates and that it does not see the economic fortitude to do so. But Fed officials, in their statement and forecasts, signaled they expect a better economy and that they could hike rates one or two times this year.
Greece also was a factor, as traders were generally optimistic it would reach a deal with its creditors.
"I'm very concerned we're going to have some sort of a corrective move here. Basically, what drives those is everybody who has access to a price chart, which is everybody, could see this as a good place to take profits off the table," said Kosar.
But Dan Greenhaus, chief global strategist at BTIG, said with the Fed still unclear on when it will boost rates, the market's bias is to go higher. He said unanswered are questions on timing of the first rate hike and how many increases there could be this year.
"I don't think the (Fed) statement or the press conference resolved any of the issues that investors are wrestling with," he said.
Stocks have been stuck in a sideways push and pull, as traders worry about the potential impact of Fed rate hikes, a Greek default, and possibly sluggish earnings.
The Nasdaq has outperformed the other indexes, benefiting from gains in technology and biotechnology, but it is an entirely different market than it was at the turn of the century.
Back then there were many stocks with P/Es in the triple digits. Yahoo's valuation, for instance, was even higher, with a price-to-earnings multiple of 1,780, compared with close to a 6 P/E today. The highest P/E on the Nasdaq currently is Autodesk at 174, versus its 130 P/E in 2000.
The S&P 500 on Thursday was up nearly 1 percent, but was still a good 15 points below its all time high of 2,134. The Nasdaq in the past year has gained 17 percent, more than double the 8 percent gain of the S&P. The Dow was about 225 points below its high of 18,351 Thursday, but the Russell 2000, like the Nasdaq has broken into new territory.