That's a very good question. I don't feel it as useful every time calling "this". You should use the keyword "this" only when it is require.
Suppose, in the following case:
public class Person
private string fullName;
public Person(string fullName)
this.fullName = fullName;
Though it is not a very good idea to use the same name but if you want to keep consistency, in such case you can use "this". In other case, it is not require at all.
If you use "ReSharper", there it doesn't recommend you to use the "this" keyword in all places. Just think about it and ask you "Why should I use something which is not useful?"