Mr. Reagan's cheerfulness

The President had called the press together quickly to make the most of the news that the unemployment rate had dropped. ''Today millions of Americans can take heart,'' he said. ''Unemployment has finally started down.'' Joblessness has peaked, he added, and would not again rise to the post-depression record of 10.8 percent.

The obvious reason for the public display of rejoicing by the President was to tell the public that his economic approach was finally paying off. He had pretty much licked inflation and there were clear signs that his initiatives were bringing down unemployment.

Beyond using this opportunity to elicit continued public support for Reaganomics, the President was sending a signal to Congress: that the good news would strengthen his resolve to carry out the highly contested elements in his domestic program; namely, further deep cuts in social programs, a big buildup in defense, and the final increment of the income-tax cut that is due to go into effect in July.

In previous days there had been hints from those around the President that he might give some ground on these positions. Now these same insiders say that the drop in the unemployment rate - together with new indicators showing the recession lifting faster than had been anticipated - has so strengthened Rea-gan's hand that he will be much slower to agree to any compromises. In fact he might even change his mind about pushing for that conditional tax increase he has proposed.

But another motive lay behind the President's quick move to hail the good news about unemployment.

Mr. Reagan is not, as some of his critics have alleged, simply an upbeat personality who likes to talk about the good and ignore the bad. He is no Pollyanna. He will, his associates concede, tend to look at a glass with water reaching the midpoint and see it as half full and not half empty. But they say he never closes his thinking to the severity of a problem. He just doesn't allow himself to be overwhelmed by it.

In recent months the President has been deeply troubled by the magnitude of unemployment and the hardship it is causing so many millions of people. But he is pictured as a man who believes that a president who never gives up the ship can help, just by this hopeful approach, bring about changes for the better.

Franklin D. Roosevelt was Reagan's boyhood hero. And Reagan is known to be convinced that FDR's confident demeanor and hopeful rhetoric were basic to the lift in the mental outlook of Americans that brought about the end of the Great Depression.

So now Mr. Reagan hails an unemployment figure showing only a slight decline. But he insists the trend in joblessness now is down. The economy now, he emphasizes, using a phrase he has used before, is definitely ''on the mend.''

Thus he believes, again with reference to FDR, that his words of hope and expectation can, as one aide puts it, ''take on a life in themselves'' in bringing about a public attitude that is more hopeful, more expectant of further economic good news, and more inclined to buy and invest and thereby stimulate the economy.

So Mr. Reagan wasn't gloating when he caused reporters to run in from all directions for a hastily called, informal press conference. He was indeed happy with the favorable development, one he had not expected. He was seeking to make political hay out of it, yes. But, more basically, he was using it to try to hasten the end of the recession.