Triangles are my favourite shape
Three points where two lines meet.
That is not a full triangle.
It can be interpreted as both an incomplete triangle and the delta symbol.
He’s very sarcastic at this point (esp after talking about arguing). It’s not his favorite shape and that it sucks to be in a triangle where the love is one-sided. There’s also a period of change (hence the delta symbol), and he’s still being sarcastic about how he loves this change
Newman is referring to a love triangle, wherein each point is a person — the woman, the lover and the cheater. The woman is split into two other points — the lover and the cheater as she is a lover when she is a cheater to her husband when she is with Newman but also a lover, thus she is torn, between love and commitment. Each point has two lines, connecting it with the others, implying that all of the members of this love triangle have some sort of connection to one another.
To help improve the meaning of these lyrics, visit "Tessellate" by ∆ (Alt-J) Lyrics and leave a comment on the lyrics box