为什么 ReactJS 没有在 history.push 上渲染组件？

为什么使用 history.push 推送新的 /user/login URL 后 Login 组件不呈现？但是对于 /user/login URL 的显式请求，会根据需要呈现 Login 组件。

import './../bootstrap'

import React, { Component } from 'react'
import { connect } from 'react-redux'
import { Route, Switch } from 'react-router-dom'
import {withRouter} from "react-router-dom"

import Header from './Header'

import Login from './Login'

import Footer from './Footer'

class App extends Component {

    componentWillMount() {

        if ( ! this.props.auth ) {

            this.props.history.push( '/user/login' )

        }

    }

    render() {

        return (

            <div>
                <Header />
                <Switch>
                    <Route path='/user/login' component={Login}/>
                </Switch>
                <Footer />
            </div>

        )

    }

}

const mapStateToProps = state => {

    return {

        auth: state.auth

    }

}

export default connect( mapStateToProps )( withRouter( App ) )