The hardest plane to see is the one coming straight at you. The closing speed is fast and the object isn't moving in your visual field. It is only growing larger. By the time that growth is significant the object is almost upon you. Even with active traffic and as a result knowing where to look, you can miss the other plane until it is right in your face. Flying with active traffic is interesting and it is fun to see a pilot do it for the first time. Particularly fun to watch are egotistical pilots who brag about how they keep their eyes outside and spot other aircraft and who slam others as keeping their head inside too much. The traffic system quickly shows how many plane the pilot has failed to spot. I get a kick out of watching the traffic system track a plane as it goes by and the pilot never sees it - well as long as I'm not the pilot missing the plane.