# Why do charges result on atoms when coordinate bonds are formed?

I have not been able to get my head around it yet.

For instance in the reaction between ammonia and borontrifloride why is it that a positive and a negative charge results? Is it that ammonia donates is lone pair completely? If not so, if sharing of lone pair donated by ammonia occurs why then should charges result?

There is little correlation, if any at all, between the formal charges assigned to two atoms that have formed a coordinative bond and the actual charges observed in the compound. Take $\ce{H3N-BH3}$: Nitrogen is so much more electronegative than boron, that the electrons are still closer to the nitrogen. Thus, nitrogen has $\delta ^-$ while boron has $\delta ^+$.
So where does the formalism come from? When drawing Lewis structures, after having filled in all bonds and lone pairs, check every single atom and mentally cleave all its bonds homolytically. Count the electrons, subtract the amount it should have as a neutral atom and the difference is its formal charge. Boron would have four electrons if all four bonds be cleaved homolytically, but it should only have three, so it gets a $1-$ charge.