what is the reason for hot and spicy dishes in hot countries? is it
because of health aspects, or to cool down the body temperature,
or...?

Hello,

Zenyany may be partly right. According to a 1998 article in a biology
journal, researchers found that people in hotter countries do eat
spicier foods than their temperate counterparts. The researchers
surmised that the spiciness actually inhibits spoilage, which is a
bigger problem near the Equator.

Source: Dave DeWitt, "Why Cooks Spice Up Their Food,"
http://www.fiery-foods.com/dave/spices.html.

Cornell News, "Spices Too Hot for Microbes," 4 March 1998,
http://www.news.cornell.edu/releases/March98/spice.hrs.html.

A follow-up study confirmed a logical outgrowth of the theory: Meat
dishes are spicier than vegetable dishes because vegetables don't
spoil as quickly.

Source: Cornell News, "Review of Recipes Finds Missing Spice Link," 26
July 2001, http://www.news.cornell.edu/Chronicle/01/7.26.01/veggie_spice.html.

I hope this answer meets your needs. If not, please request clarification.

Search strategy:
spicy food hot countries
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Jennifer Billing Paul W. Sherman 
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I have been told (although I don't know if it's true), that spices
were used to cover the taste of meat that may have spoiled in the heat
without refrigeration.

A key point here is that spices inhibit the spoilage of already-cooked
foodstuffs.  Food which has already spoiled cannot be made fit to eat
by spicing it heavily (this explanation of fiery curries dates from
British India, and contains an element of uncomprehending
condescension).

A more widely accepted explanation, in these less parochial days, is
that spicy food encourages perspiration, which is the body's
air-conditioning system.  As perspiration evaporates, a natural
cooling process takes place...similar to the canvas water bags which
cool a hiker's drinking water by means of evaporation.