Why can't unimolecular reactions be 2nd order?
A first order reaction involves the behavior of individual particles, usually their decomposition into smaller, more reactive species.  The rate of such a reaction relies solely on the number of particles present, so if we double the number of particles then the rate doubles simply because there are twice as many particles available to decompose.  

A second order reaction involves the interaction of two particles with one another. (Even if both particles are the same it is still a bimolecular reaction.)  If we double the concentration of either species we double the likelihood of a collision between those species.  If both species are the same (ex: $\ce{2A -&gt; A2}$) then the result of doubling the concentration is a quadrupling of the rate because we simultaneously double the number of particles that can interact and the number of interactions that can occur.

An metaphorical example involving people:

Unimolecular:
A couple can spontaneously break up into two separate people because of its internal properties.  (Assume there are no intercouple issues involved here.)

$\ce{C -&gt; M + W}$

If we double the number of couples we would expect breakups to occur twice as often because there are twice as many couples to be broken up.  However, since there are no intercouple issues in this example, adding additional couples to our consideration does not increase the likelihood of existing couples breaking up.

Bimolecular with Two Different Species:
A man and a woman can join together to make a couple if they collide in the right orientation with enough energy. :-)

$\ce{M + W -&gt; C}$

If I increase the number of either men or women, it is more likely that I will form a couple simply because there are more interactions between men and women occurring.  Each species' increase is first order though, since they require a partner of the other gender to couple with.  i.e. Doubling the number of women only doubles the rate at which couples are formed because they still have to connect with a man, and the number of men has not changed.

Bimolecular with a Single Species:
Homosexual men can join together to form a couple.
$\ce{M + M -&gt; C'}$

If I double the number of men, then each additional man can interact with every other man in the room, greatly increasing the likelihood of a couple forming.  This means that not only are there more men available to make couples (the first doubling of the rate), but that the existing men are also more likely to be involved in a connection that results in a new couple being formed (the second doubling of the rate.)