It is better to smile than to weep.

为什么这里的 than 后的 to 可以省略掉, 而下面 than 后的 to 不能省略呢？

To try and fail is better than not to try at all.

    【答】数个不定式由连词连接，前者无 to 时，后者也必须省略；前者有 to 时，后者可有可无。...... It is easier to say something than (to) do it. 说比做容易。（摘自《刘毅英语语法宝典》p.422）所以，下面句中than 后的 to 可以省略：It is better to smile than to weep. 

    为了对比或强调，或为表示平衡时，每个不定式的 to 都要保留。I came to praise him, (but) not to blame him. 我来赞美他，不是来责备他的。（摘自《刘毅英语语法宝典》p.421）所以，下面句中than 后的 to 不能省略：To try and fail is better than not to try at all.