why is dna frequently found associated with proteins inside of cells?

The substance referred to as "Chromatin" in the cell nucleus (which makes up the chromosomes) is actually DNA plus a range of proteins. 
These proteins perform a variety of jobs with respect to the DNA. If transcription (making RNA) is going on, you will have RNA polymerases and transcriptional regulatory proteins (transcription factors, activators, etc.) associated with that portion of DNA. DNA gets damaged by different things (chemical attack by free radicals, ionising radiation, etc.) - so DNA repair enzymes are always there, fixing the damage. If the cell is in S-phase, it will be in the process of copying all its DNA, so DNA polymerases will be doing this. Also, there are a number of structural proteins, like histones (which help "pack" the DNA properly) topoisomerases and gyrases (which help coil and uncoil the DNA), and similar. 
*Lots* of proteins are found in the nucleus.