Love is reason > Love and reason are same / love equals reason. ( not too correct to use this ,if I know what you mean
Four is 4
Love is A reason > love is ONE of (many) other reasons.
"A" points to one of many SIMILAR objects..
A dog came to the house > the writer just mentioned that it was a dog , without telling which dog it is .
love is the reason> love is ... THAT reason.
"The" points to one specific object (love)
The dog came to my house > it means a certain dog(the object) that the writer is referring to . If you dont know , then you should ask "what dog are you refering to ?"