Thank you all for your reply.
@ranochigo : When I send bitcoins to anyone, I don't mess with the fees.
Could you guide me how to set the fees correctly so that when I send bitcoins, it will "always" be in High priority ?
Thank you all again.
By default, Bitcoin Core 0.10+uses a mechanism which estimates the fees needed for x blocks before confirmation and it assumes that you don't need the funds immediately hence they would introduce a slower fee. They
To adjust the fees, go to Send tab,
and you should see this:
Click the the choose... beside the transaction fees to choose the specific transaction fee.
You can adjust the slider based on how much block you want it to confirm. It isn't fully accurate however due to the fact that miners can reject your transaction.