Unless you like constant muscle strain and back pain, it is advisable to pack as little and lightly as possible. The saying goes, "Pack everything you think you will need,  and get rid of half of it."  You can save yourself the bother by writing out a well thought out packing list.

How much and what you pack varies with the time of year you plan to travel (common sense should lead you in the right direction--assuming you have common sense). For example, a person backpacking in the winter is not going to need a swimsuit, shorts, or sandals (unless you are in fact going to a tropical locale) and a person backpacking in the summer won't want a wool sweater, a heavy coat, or thermal underwear.  People backpacking in the spring and fall should be prepared, to at least some extent, for both warm and cool weather--but of course this all depends upon the length of your journey and the climate of your travel destination.