Hi IvyGrad,
Jay pointed you to a MS OfficeOnline article on how to change the spacing.
As to the 'why', to add a bit to Jay's reply

, it's a bit more involved than just the MS designers being the ones who found it more readable <g>, but in the end, someone made a decision <g>. The question as to is it more readable is, in some respects is similar to other the long standing 'debates' over things such as:
-when or if there should be 2 spaces or 1 space after a period in a sentence based on if you are or aren't using proportional spaced fonts
- which is more important - less filling or great taste
In addition to the article that Jay mentioned, there is another article that is on the Office Online site that links, in turn, to a blog article by one of the MS Word Program Manager's (the pictures on a number of the Word team blog entries seem to have 'vanished' <g>), that in turn leads to the MS Font team blog, and that to another blog article
If you're interested here are the links to those articles on the line spacing background:
http://office.microsoft.com/en-us/word/HA102310271033.aspx
http://blogs.msdn.com/joe_friend/archive/2006/05/22/603653.aspx
http://blogs.msdn.com/fontblog/archive/2006/05/17/600507.aspx
http://blogs.msdn.com/fontblog/archive/2006/05/03/589274.aspx
You may note that in the Font team blog, 1.15 spacing wasn't one of their choices <g>, but then the readability was for 'reading' not typing ease it would seem, as well
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Word defaults to 1.5 lines spacing. Why?
I want it to default to singe spaceing.
Thanks>>
--
Bob Buckland ?

MS Office System Products MVP
*Courtesy is not expensive and can pay big dividends*