Of course the external reason they look alike is to motivate Yuna to go on her quest, which is a big coincidence upon which to hang the entire plot.

Back to internal reasons my take on it is many of the Fayth on Mt. Gagazet knew Shuyin personally, which influenced their dreams eventually reforming him physically as Tidus, while his memories existed separately in the Pyreflies in the Den of Woe. Outside of the Farplane Shuyin has no physical form other than Pyreflies. I'm not ruling out a catalyst that somehow triggers Tidus' formation though.

It reminds me of the duality of Fei and Graphf in Xenogears.