don't login as root

[Georges Heinesch <ghmlist%ibm.net@localhost>]

When logging in as root, the following message appears:

----- cut here -----
Don't login as root, use the su command
----- cut here -----

Why should I not login as root (as opposed to logging in as user and
making su to switch to root)?

.profile and .login contain this message.

--
Cu  Georges Heinesch, Luxembourg
    geohei%ibm.net@localhost - geohei%geocities.com@localhost
    http://www.geocities.com/yosemite/2480
    PGP 2.6.3i / 5.1i public key on request and on public servers

... because water just isn't wet enough !