Why does my dentist grind my tooth instead of lengthen it to match the other tooth. chipped one front tooth in two places, ground it down and bonded?

Hard to say: It will be next to impossible to comment on treatment that was done over the internet, but what you state sounds reasonable. There are many ways to address a chipped tooth and the best treatment depends on a variety of factors like the size of the chip, your occlusion, the condition of adjacent teeth, etc. Composite bonding is a very common tx option. Sometimes a tooth needs to be roughened first.

Grind or Add to: It sometimes depends on how much of the tooth chipped. If it is a small amount then grinding it down to match the other one and polishing it results in a excellent result. Sometimes it is hard to match the shade just right and you will see that color change between the composite the the natural tooth. And if it a small amount it may chip off again.

A possibility....: This is probably related to your individual bite. I'm sure you don't have an ideal bite & there is a reason why the tooth chipped in the first place. The teeth most likely do not occlude with the opposing tooth in the same way. Adding bonding to the tooth would most likely not hold & only chip again, especially since no bonding is as strong as natural enamel. Sometimes it’s best just to smooth it.