would like to/want to

would like to/want to
1.They should be allowed to practice their hobbies as much as they want.
2.They won't allow him to train as much as he would like to.
3.You can keep quiet if you want to.
为什么2,3后面有to,而1没有?是否可以加上
4.He always thinks more of himself than of others.
第二个of 是否可以省略.
还有类似的and 连接的to ,为什么有的句子前后两个to,而有的后面省略了.