First of all: The verb verderben does NOT require an object in dative case:
It can come with just the subject alone:
Die Milch verdirbt.
The milk spoils.
Or it can come with the subject and an complement in accusative case (an accusative object):
Zu viele Köche verderben den Brei.
Too many cooks spoil the broth.
And then you also can add an optional dative object in dativus commodi, which tells you who is the beneficiary or the aggrieved party. This dative object ist not a mandatory complement of the verb verderben. You can add this kind of dative object to almost every verb to express who is affected by the action:
Der Kellner bringt den Wein.
Der Kellner bringt dem Gast den Wein.
The waiter brings the wine to the guest.
Die Lampe leuchtet zu hell.
Die Lampe leuchtet der Katze zu hell.
The lamp is too bright for the cat.
And also with verderben:
Der Chef verdirbt die Laune.
Der Chef verdirbt uns die Laune.
Literal: The boss spoils the mood to/for us.
In better English (but with different grammar than the German sentence, and also with a slightly different meaning): The boss spoils our mood.
Compare with other questions like
But it is true, that, when the accusative object of verderben is die Laune, then you usually use such a dative object to express who's mood is spoiled. So »die Laune verderben« very often goes together with a dativ object, but it is not mandatory.
Now to your actual question:
You want to say:
I do not let bad weather spoil my mood.
Here are the right translations:
- Ich lasse mir nicht von schlechtem Wetter die Laune verderben.
- Ich lasse mir nicht die Laune von schlechtem Wetter verderben.
- Ich lasse mir von schlechtem Wetter nicht die Laune verderben.
- Ich lasse mir die Laune nicht von schlechtem Wetter verderben.
- (Ich lasse mir von schlechtem Wetter die Laune nicht verderben.)
- (Ich lasse mir die Laune von schlechtem Wetter nicht verderben.)
(The positioning of the word nicht is topic of lots of questions in German.stackexchange. Just look for them. I'm not going to talk about this topic here. Just note, that the sentences that I've put in brackets are grammatical ok, but you should use them only if you wish to emphasize something really strong.)
The verb lassen is used here in a manner similar to modal verbs (dürfen, wollen, sollen, ...), but it is used reflexive, which is not usual for modal verbs.
The point is: The pronoun mir is here used as the reflexive pronoun that belongs to lassen, AND as a complement of the verb verderben in dativus commodi. But it is bound stronger to the verb lassen, so it must stand immediately after it.
Now about »schlechtes Wetter« vs. »von schlechtem Wetter«:
To avoit problems with negations, I explain it without it:
This is ok:
Ich lasse schlechtes Wetter meine Laune verderben.
I let bad weather spoil my mood.
Note, that now you have two accusative objects in the sentence:
- schlechtes Wetter (Wen/was lasse ich etwas tun? Who/what do I let do something?)
- meine Laune (Wen/was verdirbt das Wetter? Who/what is spoiled by the weather?)
So, schlechte Laune belongs to the verb lassen, and meine Laune belongs to verderben.
But normally you don't say
Ich lasse meine Laune verderben.
in German. Normally you say:
Ich lasse mir die Laune verderben.
so that you have this mir again, which snaps to the verb lassen, since when it is there, it will be interpreted as the reflexive pronoun of lassen. So you might say:
Ich lasse mir schlechtes Wetter die Laune verderben.
Grammatically this is ok, but now you have three objects (one in dative case, two in accusative case), and you have two verbs (lassen and verderben), and not it becomes hard to decide which object belongs to which verb.
This becomes much easier when you replace one of the accusative objects by a prepositional object, i.e when you replace »schlechtes Wetter« by »von schlechtem Wetter«:
Ich lasse mir von schlechtem Wetter die Laune verderben.
Because now the sentence is much clearer and easier to understand.