This is a good question. It might be easiest to talk about what discrimination is first. Discrimination is generally when a group in power or in the marjority favors people of its own group at the expense of a group without power or in the minority. Reverse discrimination is when a traditionally majority group is put in some sort of disadvantage and the minority group is favored.
In the end, there is something unfair going on in both situations.
The reason why both types of discrimination is harmful is because decisions and benefits are being given out not on the basis of merit or anything equitable.