Grass is greener in Spanish
Is there an equivalent saying in Spanish that carries the same meaning as the saying in English, "The grass is always greener on the other side" (meaning something like, we always want what we don't have, or that possibilities of what could be always seem more promising than the reality of what we have)?

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Answer

Hi Alexandra, with respect to sayings, there are a lot of different translations, but my closest one (for me) would be:

"El pasto del vecino siempre es más verde que el propio".

In any case, I encourage you to visit this website: https://cvc.cervantes.es/lengua/refranero/Busqueda.aspx
It's a "multilingual saying's directory". You can look for English sayings with keywords. For example, I have found this one with the keyword "grass". Not all the translations are really helpful, but the explanations worth it.

I hope it may help you!

Answer

As a literal translations we could say: 
La hierba es siempre más verde en el otro lado 
El pasto es siempre más verde del otro lado de la cerca

Some you can hear:
Nadie está contento con su suerte.
Ver más en el plato del otro.

Answer

Maybe the closest one is:

- La  hierba crece más alta siempre en el otro lado de la verja. (New)
- (The grass grows higher always on the other side of the fence.)

but there are some similar ones:

- Gustar de lo ajeno, más por ajeno que por bueno. (Classic)
- Caballo ajeno, ni come ni se cansa. (Classic)

Cheers,
Ado

Answer

There is another ones, for example: "Culito veo culito deseo". That means that we always want anything we can see. 
And "Vacas ajenas dan mejor carne". This one means that we always think is better whatever that your neighboor have

Answer

That depends a lot on the country. one option is -Siempre está mejor el patio del vecino.-