Wheel of life and bodhicitta Why is the desire/attachment in the middle of the
Wheel of Life considered negative and the desire of bodhicitta considered positive?

What is the difference between them?
You can think of this in terms of addiction.

For the most common example, let's take addiction to alcohol. The more you desire alcohol and the more you let your desire drive your action, the more you sustain your addiction, which makes you helpless and creates all kinds of trouble for you and others.

Bodhicitta is like desire to stop drinking. You desire to be free from the addiction. Which means you desire to be your own master. You develop the will power to abstain from drinking. You learn to resist the temptation. Moreover, you learn to generate the same type of relaxation and joy that you used to get from alcohol - through other, more healthy means.

The same idea is at play in Buddhism - except in Buddhism the context is much broader, we are not talking about any specific addiction - we are willing to be free from them all, any dependency on anything whatsoever. This is why it's called Liberation, being not bound, not dependent, not addicted, not controlled, free. 

The state of mind when you are determined, intent on, convinced, motivated - to be free - which means being actively determined to make every action in context of the four right efforts - is called relative bodhicitta, while the state of having attained Liberation, or rather, mastered it, which basically means having completely mastered the four right efforts - is called absolute bodhicitta.

Now, somewhat counterintuitively, attaining liberation is not always done through passive means like letting go and acceptance. It also requires mastery of will power - particularly mastery over ones thoughts, emotions, mood, motivation, and energy. Not just mastery in the sense of being able to stop them and suppress them or accept - but in the sense of being your own power generator, your own mood maker, your own sustenance. It's the four right efforts, not the two right efforts - this is a very important point!

So these are two very different desires - one is the desire of weakness and addiction, to get your satisfaction from the outside - the other is the desire to be strong and not dependent.