Declarative sentence is just making a simple statement. It always ends with a period.
For example: I love you.
In this sentence all you're doing is just simply stating you love someone.
On the other hand exclamatory sentences emphasizes a statement. It needs to said with strong emotions. It always ends with an exclamation mark. That you know when a sentence needs to be said differently.
For example: I love you!
I would have to say this sentence with energy and excitement within me because it has an exclamation mark at the end of it.