Why are Kohanim not allowed to become tamei meis?
The Kohanim are the teachers and spirtual guides of the world. If it were permitted for Kohanim to become tamei meis then a custom would have developed to have a kohen present when a person passes from this world. This would have engendered a feeling of hatred for life. People would start believing that only to die is holy - but to live is an unfortunate reality. The Torah doesn't want us to come to such extreme attitudes, so it forbade a kohen from becoming tamei meis [and thus he is not allowed to be present when someone dies].
Love life and living!!!