Why did the british feel it necessary to impose new taxes on the american colonists

They knew very well that new taxes were going to irritate the colonists.  But they were forced to take the big chance, because Britain needed the money so badly.
This was common for a mother country to do to its colonies, and it went on for a long time.  They didn't accurately estimate the mood and character of the people in their colonies, and they just pushed it a little too far ... past the point where enough colonists would object strongly enough for long enough, and eventually decide, by a very small margin, in favor of revolution and independence.