There are several possible reasons.
1. A drying environment, e.g. hot, dry weather conditions, air conditioning, being stuck indoors or in a car with the heating on!
2. Job or leisure activities that involve long hours staring at a screen. Screen work reduces the rate at which we blink, and this reduces the amount of moisturising that goes on. This is like not using your windscreen wipers to slosh water over your windscreen, letting it get mucky and dry.
3. And on that note, driving also reduces our blink rate, so long hours of driving can have a similarly drying effect.
4. Allergic reactions such as hayfever can cause eyes to feel dry and scratchy, due to the effects of histamine.
5. Some autoimmune conditions affect the way that the eye lubricates itself. Your doctor can diagnose this type of condition.
6. Sometimes tear ducts become blocked, for a variety of reasons that need to be investigated by your doctor.
If you are suffering from dry eyes and irritation, I would recommend using A.Vogel Moisturising Eye Drops which contain Euphrasia and hyaluronic acid. These can soothe dry eyes and relieve itching, scratchiness and irritation. They are safe to be used alongside contact lenses.