I personally think cel collecting is addictive 
because:

1.  Cels are original works of art created by the 
animators.  Original artwork tends to be 
expensive, but anime cels tend to be affordable 
when compared to other media, such as Fine Art 
Oil Paintings.

2.  Production cels are unique.  Two collectors 
can not own the exact same cel, although they 
could own cels from the same sequence.

3.  Obtaining cels that are unique and in demand 
is a competitive activity.  I think it's 
satisfying knowing that you obtained something 
that everyone else wanted, but you "won".

4.  The Internet has made it possible for 
collectors to buy/sell/trade and show off their 
cels.  I would imagine that the Internet has been 
the one of the driving forces behind the growth 
in the hobby, together with the popularity 
explosion of Anime.

5.  The cels are laced with coke.