Here's why:
* Shared origins: Both languages evolved from Vulgar Latin, spoken in the Iberian Peninsula.
* High mutual intelligibility: Many words, grammatical structures, and sentence patterns are shared. A Spanish speaker can often understand basic Portuguese, and vice versa.
* Close geographical proximity: Portugal and Spain share a border, leading to historical and cultural exchange.
While other Romance languages like Italian, French, and Catalan also share similarities with Spanish, Portuguese is the closest relative, making it the most similar.