E-liquid can be harsh for a number of reasons:
Some e-liquid is harsh when freshly mixed and then smooths out as it steeps.
E-liquid can become harsh if your coil or wick needs changing.
Sometimes certain flavourings can be perceived as harsh by some people. Citrus flavours are common for this. Often switching to Nic salts or adding a little extra Vegetable Glycerine solves this problem.
If e-liquid is left in direct sunlight or somewhere hot, often it can lose flavour and become harsh.
If you have added too much nicotine or the nicotine has passed its best by date.