Your very thoughts are contradictions.
Day in, day out, you change how you think and change again.
Go with this thought and then say no to it.
Opt for a certain plan and then say it's a bad plan.

So on and so forth, the contradictory life exists.
But why?
Is it the need to have conflict that some believe is at the heart of the Human psyche?
Or is it merely the search for an understanding without realization that it is occuring?
A sub-concious push after the truth itself?

I don't know..

change... change spurs action.

That's not contradiction, that's constant re-evaluation.

Opting for a certain plan and then (at an implied later point in time) saying it's a bad plan is re-evaluating your decision in the light of later experience, events or reflection.

Opting for a certain plan which you know (at the time of opting) is a bad plan may not even be a contradiction as such - just stupidity or necessity.

I'm reading 'Drawing on the Right side of the Brain' and it's just discussing the fact that both sides of our brain think completely differently (of which I'm sure you are aware). The phrase 'on the other hand' relates directly to the other perspective considered by the left hand or 'right brain'; the reason why we second guess ourselves, or have a 'gut feeling'. I think this goes a long way toward understanding why we are full of contradictions before we come to conclusions. As Laz says 'we re-evaluate' on the information that comes to both minds.