It is very easy to say that shining armor is good at making shields, if that massive shield in a Canterlot Wedding wasnt enough proof, his cutie mark expresses it very well.

But how did he even get it? When did he know protecting ponies was his talent, and how did he even learn this spell?
Did he get it late in his life time, or did something crazy and life changing happen when he was a foal?
(Note he had his cutie mark in the flashbacks in the BBBFF song, so he did not get it during the season 1 premiere)
Was his cutie mark related to a physical feat he performed, or was it more of a symbolic cutie mark that he decided to uphold with skill later on?

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Well, there is this one comic that I really love that has a good explination

I like this one, helps to build Shining in Twi's eyes and impress that cute Alicorn. Hope he get together with her one day

Though I'm imagining the convo going

Cadence: I could've taken them

Shining: I know

Cadence: You didn't need to

Shining: I wanted to.

As for me, I imagine it to be like this. Protecting his little sister from something bad, keeping her safe and the moment he realized that was good at that, was the moment he had gained his talent

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Cool! Also, the picture wont show because you sent the link to the deviantart page, instead of the picture's seperate URL when you right click and press "Open Picture in New Tab" or something like that.

I was also thinking that Shining got his cutie mark in an act of selfless protection. If, not after learning to stand up for himself when he is being bulied.

One thing that really gets me about Shining, is that he could have helped protect others, and helped in any situation, and in any point in time before season 1. Weather it is from some bullies, or mabye a bear attack on a vacation trip.