ajax刷新的元素里jquery绑定事件失效

<div class="box box-widget">
                            <div style="display: block;" class="box-body">

                                <p>邀请了微软office负责人演示[拜拜]</p>
                                
                                <div>
                                    <span class="text-muted">Thu Sep 10 17:03:57 CST 2015</span>
                                    <span class="text-muted">0转发</span>
                                    <div class="pull-right">
                                        <input type="hidden" name="weibo_id" value="3885571242155022">
                                        <button class="btn btn-box-tool dbtn"><i class="fa fa-times"></i></button>
                                        <button class="btn btn-danger btn-flat btn-xs bbtn "><i class="fa-hand-stop-o"></i>拉黑</button>
                                        <button class="btn btn-success btn-flat btn-xs cbtn "><i class="fa fa-comments-o margin-r-5"></i>0评论</button>
                                    </div>
                                </div>

                            </div>
                            <div style="display: block;" class="box-body box-comments">
                        <!--  ajax从后台取回来的部分 -->
                            <div class="box-comment">
                                    <!-- User image -->
                                    <img class="img-circle img-sm" src="http://tp2.sinaimg.cn/3888714809/180/5727878850/1" alt="user image">
                                    <div class="comment-text">
                                        <span class="username">有人说刘伟楠这个<span class="text-muted pull-right">Sat Sep 12 16:41:23 CST 2015</span>
                                        </span>
                                        <!-- /.username -->
                                        测试测试测试
                                        <span class="text-muted pull-right">
                                            <input type="hidden" name="comment_id" value="3886290342368683">
                                            <a href="#" class="link-black text-sm rbtn"><i class="fa fa-share margin-r-5"></i>回复</a>
                                        </span>
                                    </div>
                                    <!-- /.comment-text -->
                                </div>

                        <!--  通过开关控制打开关闭，同时后台获取<div style="display: none;" class="box-body box-comments"> -->
                                
                                <form class="form-inline" action="#" method="post">
                                    <div class="input-group">
                                        <input type="hidden" name="weibo_id" value="3885571242155022">
                                        <input name="message" placeholder="Type Message ..." class="form-control" type="text"> <span class="input-group-btn">
                                        <button type="button" class="btn btn-primary btn-flat sbtn">发送</button>
                                    </span>
                                    </div>
                                </form>

                    <!--  /ajax从后台取回来的部分 -->
                            </div>

                        </div>

为什么对<a href="#" class="link-black text-sm rbtn"><i class="fa fa-share margin-r-5"></i>回复</a>绑定的时间会失效了？单纯静态网页的时候是可以触发动作，使用了动态刷新就会失效。这是什么原因？jquery只对页面载入的部分元素进行选择？

$(".cbtn").click(function(){//打开评论列表
    var boxComments=$(this).parent().parent().parent().parent().children(".box-comments");
    var weibo_id=$(this).parent().children("input[name='weibo_id']").val();//获取同一级表单的值
    $.ajax({
        url:"comment/index",
        method:"POST",
        data:"weibo_id="+weibo_id,
        success:function (content){
            boxComments.html(content);//评论框内容，后台渲染模板
        },
        error:function (content){
        
        }
    });
    boxComments.toggle('fast');
});