Q. Why I can not drag cell contents in Excel sheet?

I have come across with similar problem many times in the past and for some reason I can't remember how to solve this problem. This time when I was working on data creation using Excel, the same problem occurs.

I tried searching on the web but didn't find even a single site where there is a reason mentioned clearly about why all of a sudden this happens.

Why this option gets disabled automatically in excel sheet ? Anyone of you know the reason this WHY then please share it...!