There's an actual answer to this, Sudeep. It can be traced to one man: Noah Webster.

The United States broke with England in 1776. In the early 1800s we were a proud country, anxious to establish a distinct national identity. We were already a literate country--the American Revolution was an early example of "social media." It was sparked by Thomas Paine's writings, and organized through the good paper mail system of the era. We already had printers and publishers.

Webster saw a good marketing opportunity and successfully wrote and launched, first a spelling textbook for schools, then An American Dictionary of the English Language. He established himself, personally, one man, as the U.S. authority on words and spelling.

(In contrast, the Oxford English Dictionary was the product of a well-organized team, and Simon Winchester wrote not just one but two terrific and readable books about it: The Professor and the Madman, and The Meaning of Everything.)

Most of the annoying, irritating, useless differences between British and U.S. spelling exist just because Noah Webster thought they were a good idea. They are all in the direction of simplification and being more phonetic, but they didn't go far enough to help, they only went far enough to create a problem.

You can blame Webster personally for colour/color, and you can also blame him for center/centre and traveller/traveler.

I've visited the Mark Twain House in Hartford, Connecticut several times, but I've never gotten around to visiting the Noah Webster House--another house museum in Hartford--and I probably should someday. But, what can I say? The Mark Twain house is bigger and more famous, and I love Mark Twain much more than I love Noah Webster.

The difference between U.S. and British English is wildly exaggerated. If you were just looking at the language as spoken, you wouldn't draw a big thick line down the center of the Atlantic Ocean. There are two reasons we think of British and U.S. English as "different," but Maine and Texas English as "the same."

1) "A language is a dialect with an army and a navy," i.e. we tend to think we see differences where we see a national border;

2) Because of history and tradition, we use different dictionaries and have independent publishing houses. (Or used to. I guess it's all Bertelsmann nowadays). So, books published in the UK get new titles (Harry Potter and the Philosopher's Stone becomes Harry Potter and the Sorcerer's Stone), and words get changed--nappies to diapers, boots (of cars) to trunks, carrier bags to shopping bags, and so forth.

It was very funny to me to read a British edition of John Steinbeck's The Grapes of Wrath. Steinbeck is imitating spoken speech and has his characters talking about "a gallon a gas." In the British edition, the Okie migrant workers talk about "a gallon a petrol." As if Tom Joad were played by David Niven in a British accent.

To Susan about the deliberately changed spelling.

I recall a story about it:

<font size="2" face="Arial">A monk in the monastery tearing off the patches of his hear and howling in despair. <o:p></o:p></font>

<font size="2" face="Arial">- What’s up? – another guy asks.<o:p></o:p></font>

<font size="2" face="Arial">- Finally I have come across the original script and found out that they had TERRIBLY changed one word for all other next copies!<o:p></o:p></font>

<font size="2" face="Arial">- What’s that word?<o:p></o:p></font>

<font size="2" face="Arial">- They had put it ‘celibate’ instead of the correct ‘celebrate’ !!!<o:p></o:p></font>

I read in an article online that when an American first made the Webster´s dictionary, he deliberately changed the spelling of some words in order to make more of a distinction between American English and British English.  Webster´s dictionary became the first dominant reference for spelling in the English language. Since you asked this question, I took a quick look around to see if others agreed that this was the source.   

It could be wrong, but this wikipedia article also supports the view that Noah Wester could be credited with these spelling differences.

https://en.wikipedia.org/wiki/Webster's_Dictionary

I guess he could have been U phobic.  I think it is more likely he just thought the U was unnecessary and thought it more efficient to drop the U. 

one main difference that  makes me curious to know the mystery behind it!!! Do you think American English is U-phobic and British English is U-philic?? For example:-- 

Am-Eng                          Br-Eng

Color                              Colour                

Flavor                             Flavour

Humor                            Humour

labor                               Labour

So could you please tell me why is this so?  I've my own guess but that may not be correct or may sound stupid:)