I need a good excuse vs. I need a good reason

Hey everyone, do you guys know the different nuance between these two sentences?

I need a good excuse

This means that you WANT TO DO something but you need a pretext (託つける - かこつける) to do it. 

A: I want to see what Terry's new girlfriend looks like.

B: She's at his place right now just go over and say hi.

A: But I need a good excuse to go over there, if I just suddenly show up it'll look strange.

B: Why don't you return that book you borrowed?

A: Good idea! That's a great excuse.

I need a good reason

This means that you DON'T WANT TO do something so someone has to persuade ( 説き付ける - ときつける) you do to it. You might also hear people say "give me a good reason".

C: I need your help.

D: Why should I help you? You're always mean to me~!

C: Please, I really need to pass this test.

D: Give me a good reason why I should help you?

C: If you help me pass I promise I'll stop being so mean to you.

D: That's not good enough, how do I know you'll keep your promise? Try again, I need a good reason.

C:  OK, I'll apologize to you in front of the whole school will you help me then?

D: You got a deal!

Any questions?

Have a good week,

Phil