Not just the gains of the recent years speak against selling now. The "sell in May" suggestion conflicts with logic. If there would be any simple rule to beat the market, everybody who do it. But the majority is the market and cannot beat the market because it would beat itself.
The slogan also is self-defeating. If many follow and sell in May, their sales would set the stock market under pressure and stocks would become significantly cheaper. When the slogan followers come back to the market - say by the autumn - their purchases would raise stock prices. Following stubbornly the "sell in May" proposal would therefore implement to sell cheap and to buy expensive.
And there are more arguments against selling in May. In the long run, stock prices go up. Since its start in the year 1896 the Dow Jones, another gauge for the U.S. stock market, rose 7% per year - and around 0.5% per month - on average. Thus the probability of a rising stock market is higher than of a falling.
I believe that the likelihood of rising stock prices in the coming months is very high. It seems that the U.S. economy entered a virtuous circle. The job market is healing, creating more income, and the falling jobless rates reduce the risk of losing ones job and raise the optimism of the consumers & investors. The European economy is on the way to recovery & China seems to master the soft-landing. And company profits are climbing thanks to growing productivity & efficiency.
I think that the stock market will get tailwinds from a new industrial revolution which is now going on: Rapid advances in 3D-printing, robotics, nano-technology, bioengineering (new medicaments) and more industries are working together and are reducing costs, opening new markets and making our lives better. And: The number of Internet users - and hours & money spend online - is worldwide exploding thanks to cheap smartphones and other devices which give access to the worldwide net. Thus I believe that the bull market which started in spring 2009 has many years to go.