Why is it so hard to learn a different language??

at the age of up to 3 years.. our grasping power is relatively very high. This is the reason why it is so easy to observe and learn anything and everything including language during this age.. Its the learning process of a fresh brain.. Thats why we never ever mess up the grammar of our mother tongue. So earliest will be the easiest.. (the learning period extends up to around 10 years..- Not that our IQ decreases there after.. but observation.. we mostly never forget something we learn at very small ages.. Human brain have always been a mystery..).. The only choice is.. that you need to put in more effort to cop up with this difference..