라서

Because it is (after a noun)

Gives the reason for something. It attaches to the 'to be' verb 이다 (or its negative 아니다), so what sits in front of it is a noun: because it's summer, because it's dawn, because it's an older model. The consequence is often left unsaid — three of these lines simply trail off after it and let the listener finish the thought. For verbs and adjectives the equivalent is 아서/어서, while 니까 gives a reason too but pushes it harder.

Register: neutral; both polite and casual lines here