"won't" vs. "wouldn't"

Contrary to what you seem to think, wouldn't and won't are almost never interchangeable.

The simple negative won't is used for future negative actions or for refusals.

• I won't go to the store tomorrow if it's raining. (Future negative.)
• I won't go to the dance with you. (Refusal.)

The negative wouldn't is used for counterfactual statements, and for future statements embedded in a past-tense narrative.

• I wouldn't shout if I were you. (Counterfactual)
• He said he wouldn't like it. (Future embedded in past narrative.)

In every case here, replacing won't with wouldn't results in something either ungrammatical, or it changes the meaning of the sentence.

Edit: An additional requirement for will/would is tense concord, which means that subordinate verbs in a complex or compound sentence must agree in tense with the main verbs. This applies to the two halves of an if/then construction, as well as to verbs in relative clauses. For this purpose, will is considered to be present tense, and would is past tense. So you see things like:

• He will be dead if he goes to the store.

• He would be dead if he went to the store.

• He says he will open the envelope.

• He said he would open the envelope.

In this case, the distinction between will/would doesn't carry any semantic weight, but is required by English grammar. Swapping will and would in any of the preceding sentences results in an ungrammatical utterance.

As Colin mentions, in this context would implies a hypothetical situation, whereas will implies intention or (likely) prediction.

So to take one of the examples:

If* you ever break up with your girlfriend, won't you become lonely?

...implies a greater likelihood of the break-up occurring than:

If you ever break up with your girlfriend, wouldn't you become lonely?

In the latter case, because we are talking about a hypothetical occurrence, the first part of the sentence could well be changed to be subjunctive:

If you were to ever break up with your girlfriend, wouldn't you become lonely?

...in which case wouldn't has to be used instead of won't.

(* Note: I've used if rather than should here, as should sounds rather old-fashioned when used in this context, to my ears at least.)

"Will" and its negative "won't" are about future intention or prediction.

"Would" and its negative "wouldn't" have a range of meanings, but the relevant one here is about hypothetical intention or prediction.

So "I won't go" is a statement of my actual intention.

"I wouldn't go" is saying something about my intention in some hypothetical situation.

I'm sure there are contexts when both can be used, but I think there will still be a difference in meaning.

As Kieren has commented, won't is short for will not whereas wouldn't is short for would not. They're not interchangeable. e.g. you wouldn't write I won't do that if I were you, you'd write I wouldn't do that if I were you. If you're not going to be at a party you might write I won't be there and not I wouldn't be there.