To answer the second part first is to say that the reason we do not seem to mind is because we do not understand that our sin is an offense to God. After David had slept with another man's wife he tried to cover it by getting Uriah to come home from war and sleep with Bathsheba. Uriah is a noble man and does not do it so David has him killed in order to cover his first sin by getting Uriah's wife pregnant. After we could easily say that David sinned against Uriah, David himself says he sinned against God. In Psalm 51 David is repenting of the fact that he has done this horrible thing and in verse 4 David writes, "Against you, you only, have I sinned and done what is evil in your sight." Until we realize that sin is an offense to the Holy God we will always take it far more lightly that we should.
To answer the first part of my question, however, is because we are often to passive in the fight against sin. Think back to Genesis 4 when Cain is about to kill his own brother. In verses 4 and 5 we see why we have to be active in the fight against sin, "The Lord said to Cain, 'Why are you angry, and why has your face fallen? If you do well, will you not be accepted? And if you do not do well, sin is crouching at the door. It's desire is for you, but you must rule over it.'" If sin desires us, as it did Cain, then we too must rule over it. There is nothing passive about this. Christians must actively fight against the sin that desires to overcome us and cause us to flee from God. In Romans 7:21-23 Paul helps us to see there is a war going on within us that we must actively fight.
Christians, we can cannot be lazy in our strivings to live for the glory of God. We cannot expect sin and temptation to go on vacation so we can have a rest for a while. We must, by God's grace, fight with all of our might to live holy before God and to rule over sin or else it will rule over us.