why difficulty changes so often?
Because of the hash rate that is also different at times.
it's obvious. But not every minute...
http://www.cryptocoinsnews.com/2014/03/13/dogecoin-forks-again-to-avoid-multipool-exploit/lso included in the update is the DigiShield difficulty algorithm, which will allow the mining difficulty to adjust near-instantly. This was developed by the team at DigiByte, who allowed Dogecoin to use it in a gracious show of community-spirit. In a rapidly expanding market, such team spirit is rare.
Multipools are mining groups that use their hashing power to mine whichever coin is most profitable at a given moment. Miners participating in multipools configure their rigs in an all-purpose manner and the multipool server then calculates the exchange rates of eligible cryptocurrencies, directing the massive mining power to net the miners the greatest profit.
The mercenary nature of multipools is the result of free-market competition, and in many ways is necessary for assuring that there will be hash-rate available to any coin that has demand.
However, a problem arises when a multipool causes a sudden surge of hash-rate, resulting in increased difficulty of mining. Once the difficulty rises and the coin is less profitable, the multipool moves on, leaving smaller miners struggling through difficult calculations until the next difficulty-adjustment.
Using DigiShield, difficulty adjustment is nearly instantaneous, eliminating the problem of large pools surging in when the difficulty is low and abandoning once it adjusts. This is a big deal, because this addition sets Dogecoin apart from the cryptocurrency it was cloned from, Litecoin.
Now Dogecoin has something important that Litecoin doesn't.