We should guard our souls for several good reasons. 

Satan’s goal is our destruction.

He is a predator, on the prowl and seeking your soul (1 Peter 5:8). We need to be on guard because sometimes it’s hard to tell that someone has evil intentions (2 Corinthians 11:14-15). Even spiritual leaders teach error! 
It’s also easy to be destroyed gradually by sin (Hebrews 3:12-14). It can deceive you, draw you in so that you think there is no danger, and before you know it you’re caught. 

God’s Solution

The solution is to focus on God daily and trust in His Word. If we live with a spiritual mindset and walk in the light that the Bible describes, them we will be in the best position to see Satan in disguise and sin that is creeping up on us. 

God tells us to knock, ask, and seek and the help we need will be available to us (Matthew 7:7-11).

God also set up the church as a place of encouragement (Hebrews 3:13). There we can meet other people who are also on guard and by strength of our common faith.

We are not left alone to deal with sin alone because God also gives us prayer as a way to cope with it (Philippians 4:6-7). God is not waiting to strike us down when we sin but is waiting for us to repent and even won’t let us be tempted beyond what we can bear (1 Corinthians 10:13).

God is greater than Satan. Whether we chose to tap into God’s power is up to us!