Is it obligatory to respond to invitation cards?

The Shaykh : It is not obligatory to respond to cards unless you know that the man who sent you the card with an invitation means it sincerely, because many cards are sent to people as a matter of courtesy, and it does not matter to him whether you attend or not. But if you know that it matters to him that you attend because he is a relative or a friend of yours, then respond.
The student : O Shaykh ...

The Shaykh : Yes.
The student: ...

The Shaykh : No, be lenient, be lenient; leniency is good.