If vs When?

I was watching War Horse last night and heard the sentence

I'll be there IF you need me

and it made me think about just how different that is from
I'll be there WHEN you need me


I know that most of us use these sentences interchangeably; never consciously meaning anything different.  But their meanings, to my mind, are very different. 

The first is saying "I wait to be asked to come and help you"
The second is more like "I am always here, just waiting for you to ask"
Many people would not hear or feel a difference between these, but I do; I see the difference between a completely open offer of availability and a slight step back. 

Others might see it in another way; perhaps that the first is saying "I know you will cope fabulously without me, but if you do need me don't worry, I'm here" and the second saying "I know you'll need me because you always do and I'll be around".
Isn't life and language and emotions difficult.

Of course I know that I am very in my head, years of therapy will do that to you; and that I spend time worrying and pondering on phrases I've heard and used.  But I think it's important to once in a while stop and think about the language and phraseology we use and how it might be heard.

what do you think?