After detecting the transmitted signal of a garage door opener with SDR, is it correct to assume the first repeating pattern of the signal is preamble?
It is a reasonable assumption. For very simple things like garage door openers, the lines between preamble and payload might be a bit blurry:
A preamble has the purpose of giving a receiver something "known" to look for. Now, a garage door receiver might simply just be looking for one special kind of signal and open its door if it sees that.
I only see two one and zeros with equal intervals and after that there is no pattern. Is it correct to assume that those two one and zeros are preamble of the transmitted signal?
Maybe! to elaborate:
Should the preamble always be a repeating pattern and if so why?
No, not generally, not always.
However, it makes a lot of sense to have alternating symbols in a preamble. That way, you can easily tell a preamble from e.g. a constant tone interferer, and also, and in practice very importantly, you can use the preamble to get the timing of symbols – since no two oscillators are 100% identical, it's necessary that the receiver "learns" about how long a symbol is by observing the symbol rate of the transmitter.