Why do we get bored of certain food items after repeatedly eating them?
Humans inadvertently try to balance neophilia and neophobia - fancy names for preferring the new and fearing the new. With regards to food, most humans, and animals, take an overall stance of cautious curiosity. Curiosity as in discovering a new edible food that helps them survive and thrive, and caution as in any new food item may cause harm. Certain individuals will have less curiosity, being content with just eating a few foods, while others will seek new experiences when they can. That's part of the variety in humanity.

For example, people from the Eastern countries rarely ever get bored of certain foods, as they are a primary source of fuel, such as rice or noodles. Or like people from western countries, many of them eat bacon, eggs, and toast every morning for sixty years. They eat it repeatedly, almost everyday, yet they continue to do so without complaining of boredom. 

The scientific explanation is just that the brain will get tired of the same thing. That's why do many fast foods are engineered so carefully tantalize (bliss factor) and yet not go over the point when the brain gets overloaded. This phenomenon doesn't really count as a factor of boredom, rather more like insensitive to repeated exposure. It's like when you are exposed to a strong smell (e.g. Perfume) for a longtime. The brain, neurotransmitters and olfactory receptors just ignores it and doesn't register it. It is suggested why we females keep putting more and more layers of perfume on, we just don't smell it anymore . 

Hope that helps &lt;3.