Just a note: the resistance you are getting to this question is based in part on the fact that it has been asked and answered -- and essentially ignored -- a number of times in this forum, yet the notion persists that the reasons behind it are somehow mysterious. This plays into the idea that there's no real difference between Bush and Kerry ... and ... and ... I'll let those who desire to do so follow that slippery slope.
In any case, regarding increasing troop levels, there are several reasons. Not inclusive and in no particular order:
1) Many current regular army positions concerned with modern national security and intelligence gathering needs remain empty. As far back as 2002, Kerry identified some 15,000 positions requiring specific language proficiencies which the Pentagon had not made a significant effort to fill. (While not strictly related to the size of the military, another 2,000 - 3,000 similar positions in the State Department were likewise without staff members, indicating a lack of concern among the Bush administration of the importance of these elements.) This is over a third of the 40,000 number commonly floated.
2) Of particular concern to a potential Kerry administration would be Afghanistan, which is once again rapidly descending into the kind of lawlessness that gave rise to the Taliban and allowed what government there was there to be in effect controlled by al Qaeda. This is largely due to BushCo stripping the country of even the minimal force necessary to attempt to stabilize the region.
3) Iraq will not be immediately abandoned militarily in toto unless some significant and dramatic changes occur between now and January, which is unlikely. However, with current troop levels, the reserves and national guard are unduly burdened with the duty, and regular army forces are stretched to such extremes that should there be a real emergency somewhere in the world, the ability to the American military to respond is severely limited, if not curtailed entirely. A significant portion of the proposed increase would likely be used to reduce the reliance on part-time forces and to bolster the regular army's numbers in Iraq until such time as they can be relieved according to the Kerry administration's goals.
Whether you agree with the specifics of this kind of reasoning, it differs substantially from a PNAC/Neo-Con centered agenda.
Joshua Marshall published an excellent article on this subject in a recent issue of
The Atlantic. I'm sure few here will agree with all of it and may in fact retract in horror to specific elements, but I urge everyone interested in this question to read it to the end. It's not a soundbite, rather a thorough and reasoned analysis, and it clearly describes the differences between a proposed Kerry foreign policy and the current neoconservative/Bush policy.
http://www.theatlantic.com/issues/2004/07/marshall.htm