Paper is basically an interwoven fabric of fibers, sort of like a knit sweater.
The longer the fibers, the better they weave together and the higher quality the paper. In some paper products, the fibers are already so short they can't be recycled or re-woven into new products. Certain chemical or plastic coatings do not break down in water, making it difficult to recycle some paper products. Grease and certain food residues will stain into paper fiber. Thus, even when the paper is recycled, the new product will still have stained fibers.