Question

why should we not pursue short term gains?

Answer 1

Ans to the question...

We should not pursue short tern gains because it will satisfy the management of resources for the present generation not for the future generations but when we pursue long term gains it will lead to sustainable management of natural resources.

Answer 2

Are you asking about short term gains in the financial/tax or the personal sense?

From a investment point of view, short term gains have a few disadvantages:

They are taxed at a higher rate than long term gains
Associated transaction fees may be higher, reducing your overall profits
The time you must spend make the decision and the transaction is going to be higher (proportionally) than for long term gains. (Think day trader vs. buy-and-hold, as an extreme example)
That being said, a short term investment may still have a place in your financial activities - the gain may outweigh the disadvantages or it may provide a cushion against risk.

Personal short term gains (impulse purchases, impulse affairs, impulse crimes) can also have disadvantages, but that is going to vary a good deal by situation. The first thing to consider, ironically, should be long longer term consequences: do the long term costs outweigh the short term gains?