師匠 昔気質なくせに 妙なことでやさしいから、いつも お前に 巻きこまれて...
"My teacher is old fashioned, but because he's kind, the always get involved with you..."
How is the で particle used here?
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As sundowner already pointed out in their comment, the particle で in 妙なことでやさしい is employed to indicate what the master is kind about. On the other hand, 妙なことにやさしい primarily lead us to the reading of “strangely, the master is kind”, which conflicts with the premise that the master is usually 昔気質.
About the expression ~なことに(は), please refer to past questions: Is this なことには here functions the same as 不思議なことには? and What's the difference between 意外にも and 意外に?