Why does Jesus keep company to people known to be sinners?

John 3:17 For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him.

People are not saved by good things that they do, avoiding sin, or even church attendance. People are saved purely by the grace of God. It is a gift that nobody deserves. I love how Matt Chandler syas it, "there is nothing in The Bible about being saved by our effort or religious stamina, at all, we are saved by an act of God.

Jesus spent time with "sinners" to show them the truth of who God is and to offer salvation for sin, death, hell, etc. to them. Also to show everyone else that, in God's eyes nobody is any better than the worst sinner. Romans 3:23 all have... fallen short of the glory of God.

Hope this helps.