Dry hair develops when your hair doesn't get or retain enough moisture. This reduces its sheen and can make it appear frizzy, lifeless, and dull. ... If your hair is healthy, natural oils in the outer layer help protect the inner layers. They also reflect light, making your hair appear shiny.

Heat damage
You'll know whether your heat styling tools are adversely affecting your mane -- the texture will be brittle and you'll probably have split ends.
Extreme Breakage and "Shedding"
    Your hair has definitely been weakened if you see large amounts falling out when you comb, brush or style your locks.
Dulled Shine
    Go easy on gels, pomades and sprays -- if your locks are looking lackluster, it might be a result of product buildup.
Dye Overload
    If you frequently color your hair, the hydrogen peroxide in hair dyes can leave your tresses dry and damaged.
Too Many Tangles
    Using cheap, plastic brushes rather than those with nylon bristles will tear at your head and leave your hair in a tangled mess.
Lack of Moisture
   Healthy hair is supple and soft, not brittle and dry. If you have dry hair, you might not use conditioner with enough frequency -- conditioner locks and seals moisture on your hair shaft.

Dry hair
Just like your skin, hair can dry out when exposed to the cold or changes in temperature (like when you step inside from the cold). Cold and heat can make your hair lose moisture, and water makes up 10% of hair. Your hair will feel rough and may make a crackling sound when you touch it.
Hair loss, dull hair
Have you recently gone through some tough times? Logging lots of overtime at the office? Your hair, which grows about 1.2 cm per month, can feel the effects, even weeks later.
Dull, dry, brittle hair
Dyes, perms, relaxers and other chemical treatments act in the same way a stripping agent works on furniture. The more you use them, the more your hair feels the strain.

Dull hair
You aren’t rinsing your hair properly. You don’t need to use half a bottle of shampoo to wash your hair. Too much shampoo makes rinsing more difficult and could result in residue on your hair, which weighs it down and makes it dull.

http://www.brunet.ca/en/advices/dull-hair-causes-and-solutions.html
https://www.livestrong.com/article/59728-signs-damaged-hair/