Austria/Switzerland

Real World Relations
This pairing has some historical basis, but usually only dependency.

Switzerland is the fourth-largest investor in Austrian products, and employs around 35,000 Swiss citizens in Austrian companies. Switzerland is one of the top five major importers of Austrian products, and vice versa for Austria. So they both depend on eachother economically quite a bit.

Another fact is that the Habsburgs, who ruled Austria for over six centuries, originated from Aargu, Switzerland.

Together, Austria and Switzerland also hosted and organized the 2008 UEFA Euro Football Championship, which fans speculate was a sign of friendliness.

In today's world, German is a main language of both Austria and Switzerland, which makes a connection and familiarity between the two.

Switzerland and Austria are actually close neighbors; close enough for there to be a Eurail pass specifically for Austria-Switzerland travel.

Basis Of Romantic Relationship in Hetalia
Upon first glance, Austria's aristocratic attitude and Switzerland's more violent behavior would not mesh well. Though some supporters of this pairing claim that they are tsunderes towards each other. Also, seeing as Austria is one of the Hetalia characters that has never threatened Liechtenstein at some point or another, it could be a factor that makes him more acceptable to Switzerland and his protectiveness of his little sister.