We should take care of our mental health regularly because just how our physical bodies need to be exercised and taken care of, our minds need to be exercised and looked after too.
We take care of our physical bodies by maintaining hygiene, working out, jogging etc. But what do many of us do to take care of our minds on a regular basis !?

To put it into perspective we use
shampoo for our hair,
soap for our bodies,
face wash & sunscreen for our face,
toothpaste for oral hygiene,
multivitamins for optimal body functioning which includes hair growth & skin care.
Then a gym membership for working out,
different set of clothes and shoes for training, then we spend on protein and other supplements !!

Now, compare that to how much we spend on taking care of our minds. Here's an example - 75% of all healthcare costs are made on chronic diseases which are preventable. And we as society spend less than 5% of the total cost on prevention of those same diseases. (https://khn.org/news/052411thorpelever/)

Reading and Meditation are the two most important things we can do to pamper our minds. 
And as we take care of our minds and thoughts it also has an effect on our physical bodies. In India, there is an ancestral saying "Unhealthy mind leads to an unhealthy body".