Rofl, sorry I made this comment just before I had to run back to work and...didn't actually elaborate on anything. >.>;

Yang likes challenging guys who look stronger than her, or rather stronger than she appears to be. She's actually deceptively strong because she fights monsters for a living, among other things. Shiro also fits into the wheelhouse of guys she'd think are attractive so extra incentive there for her to show off.

If you're okay with massive spoilers for RWBY I can go into a bit more about Yang and why it'd be kinda hilarious for her to arm-wrestle Shiro but I don't want to spoil anything major if it was a series you were planning on looking into at some point. ^^;