What is an eysore? And a sight for sore eyes?

English can be a very confusing language.

An eyesore is something - typically a building - which is not pleasing to look at. This building is an example: 
Eyesore: this building site may turn into a swan one day!

A sight for sore eyes is the opposite - something is which aesthetically pleasing to the eye. This is often used as form of flattery e.g. you're a sight for sore eyes

What is the origin of eyesore?

Shakespeare did not coin the phrase - though he is responsible for eyeball.  He did, however, provide an early example in The Taming of the Shrew, albeit one that is more metaphorical than is typical in modern English: 
Why, sir, you know this is your wedding-day:
First were we sad, fearing you would not come;
Now sadder, that you come so unprovided.
Fie, doff this habit, shame to your estate,
An eye-sore to our solemn festival!

And sight for sore eyes?

First recorded example comes in another giant of literature: Jonathan Swift  in A complete collection of genteel and ingenious conversation, 1738:
"The Sight of you is good for sore Eyes."

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