Originally Posted by
eshagh My RAM is 1GB
That is your problem. Basically what's happening is your apps are demanding more RAM than is available. Because of this, background apps will be killed out of necessity just to be able to keep the device running. Today's apps are a lot more resource intensive than when your device was made, and your only real option is to replace it. 4 GB of RAM is about standard on today's devices.
What you are experiencing is the same symptom that can happen with memory booster apps. That's why B. Diddy was asking that question. So if you are ever tempted by those, just avoid them like the plague because they will fight how Android is designed to operate.