Don't use comments merely to justify the grade. That is, don't just summarize all the mistakes you've marked or point out all the deficiencies in the paper so that the student won't object to the grade you assign. If all the comments are negative, the students will either not read them or be depressed by them. Remember that you want to increase their motivation to do better on future papers. Be sure to point out some positive things about the paper.
Most papers have at least a few things that the student has done well.
Sometimes you will have to stretch a little, since some papers show little thought and are grammatical and stylistic messes. Intermix positive and negative comments. It's usually best to begin with a positive comment about the paper, then mention a criticism, then another positive, and so on. The intermixture helps the student accept the criticisms and shows that you have a balanced response to the paper. The best thing to do may be to list the comments without any logical transitions.