For the same reason, avoid washing your hair on the first and last day of the New Year. It is also considered unlucky to use anything sharp on New Year's day - knives, scissors, even nail clippers. The action of the sharp blades risks cutting the threads of good fortune brought in at New Year. It is important not to use language which is negative- having an argument on New Year's day is to be avoided at all costs.
Words related to sickness and death are to be avoided.
This even extends to the use of words which sound like the words for death or sickness. To avoid any association with death, any slaughtering of poultry or livestock is carried out on New Year's Eve. Finally, care must be taken not to stumble or to break anything - this would be indicative of bad luck ahead.