Take a look at Hillary Clinton's record and her campaign. That's as rational a reason as anyone in his right mind needs. The future of the Supreme Court is another rational reason. There's one vacancy already and there's sure to be one or two more in the near future. Those replacements are going to define the court for a generation or two, and may determine the future viability of the U.S.
No, Russ, "Hillary is worse" isn't a reason why Trump is qualified, capable, or reasonable. As for the USSC, I'm always surprised when conservatives act as if they are nobly standing guard over the laws and traditions this nation when, in fact, it is the conservative wing that has gone to incredible semantic lengths to defend a lack of equal protection under the law, not to mention our rights to be secure against unreasonable search and seizure. Of course, the standard reply is that liberals are anti-second amendment, but since
Heller found an individual right to bear arms, gun rights haven't rolled back at all, but equal protection and 5th amendment rights have been under almost constant attack by a 5-4 conservative majority (albeit with Kennedy sometimes serving as a rational centrist, and Roberts occasionally deferring to the legislative branch's assumed intent).