Why do some of the verbs get tildas?
se lo quiero comprar becomes comprárselo
se lo estamos decorando becomes estamos decorándoselo
It’s to preserve the stress.
com-PRAR has the stress at the end, on the syllable “PRAR”. So the accent in com-PRÁR-se-lo keeps the stress on the same syllable “PRAR”.
The same is true for de-co-RAN-do and de-co-RÁN-do-se-lo.
Marcos is absolutely right. Just adding that when the word becomes a longer word when you add the pronouns, and the stress is now on the third before last syllable, you need to use the tilde on that syllable. These words, with the accent on the antepenultimate syllable are called "palabras esdrújulas".
Here are two useful articles to understand more about accents and tilde in Spanish. I hope they're useful.
Written accents in Spanish
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