Bhagavad Gita, Chapter 2, Verse 62

Thinking about objects, a man forms attachment to them. From attachment comes longing, and from longing, anger.

It starts with just thinking about something. It doesn't end there.

Krishna lays out a chain reaction here: dwelling on something breeds attachment, attachment breeds craving, and craving, when blocked, breeds anger. The starting point looks harmless, which is exactly what makes it easy to miss.