It doesn't, but Trump has repeatedly demonstrated that he's (a) illiterate, and (b) dictatorial. He had two years with a compliant Congress and it wasn't an emergency. Border crossings are at their lowest level in more than a decade.
No, it is not an emergency if you admit you don't have to do something. But, we ARE dealing with Trump in this respect, so you can't expect him to act rationally. He has been thwarted in his demand for $5.7 bn for his wall/barrier/steel slats/beaded curtain, so he is determined to overreact in the only way he can think of to get back at and around Pelosi. Not only will it wind up in the courts, but it will give him center stage throughout the entire court proceedings and, in his mind, distract from Mueller's investigation.
One of the things that struck me in his rambling speech on where he's going to take the funds from was his statement about the funds he plans to take from the military. He claimed it's intended use was not very important at all, so I ask - if the money was set aside for something unimportant, why is is included in the military budget?
Unless is directly benefits him, nothing is important. If you look up "narcissist" in the dictionary, Trumps \picture is next to it.
It absolutely slays him that Obama got a Nobel peace prize. He talks about crooked Hillary and the others, but Obama is the one who sticks in his craw. He has spent his first two years in office dismantling everything that Obama did - or trying.