Like it or not, we were born into a world at war. As we might say in England, "Satan does not play cricket by the rule book!" In other words, he doesn't play fair. He is an opportunist, taking every opportunity he can to gain a foothold in our lives.
A paraphrase of John 10:10 says, "The thief has come to steal and kill and destroy. I (Jesus) have come that you might have life more abundantly." Satan loves to steal from us. He seeks to steal our joy, giving us depression. He tries to steal our peace, giving us fear and anxiety, and so on.
Satan needs a legal right in order to gain a foothold in our lives. We sometimes unwittingly give him that right, or maybe we were defenceless at the time. For example, a child who is the victim of abuse may have horrendous problems, but they were not to blame for what happened to them.
Thankfully, Jesus has made provision for all these things to be dealt with through his victory over death on the cross.