For some reason he felt constrained to lower his voice.是什么意思？

For some reason he felt constrained to lower his voice.

不知怎么的，他觉得自己必须得压低声音。

for

prep. 为，为了，倾向于，关于，当作；conj. 因为，由于；

例句
I've come looking for a bit of advice.
我特来讨教。

some

det. 一些，某个，部分，相当多的；pron. 一些，若干，（数量不确切时用）有些人；adv. 大约，有点，稍微；

例句
It's not a question of some abstract concept.
这不是某个抽象概念的问题。

reason

n. 理由，原因，理性，理智；vt.& vi. 推理，思考，争辩，辩论，向…解释；

例句
As always, Peter had a reason for his action...
一如往常，彼得对自己的行为总有说辞。

he

pron. 他，它，一个人；n. 雄性动物；

例句
Logic had prevailed and he had abandoned the idea.
最终理智占了上风，他打消了那个念头。

felt

n. 毛毡；v. 摸索( feel的过去式和过去分词 )，触，深感，以为；

例句
She felt as if his words had granted her absolution.
她感觉他的那番话好似给她下了道赦令。

constrained

adj. 不舒服的，被强迫的，拘泥的；v. 强迫( constrain的过去式和过去分词 )，强使，限制，约束；

例句
Changes in temperature occur slowly and are constrained within relatively tight bounds.
温度变化缓慢，而且局限在相对较小的范围内。

to

prep. 向，朝着，到，关于，属于；adv. 朝一个方向的，到某种状态，关闭；

例句
I suggest you leave it to an expert...
我建议你把它留给专家来解决。

lower

adj. 下方的，在底部的，较低级的；v. 降低，减少，缩小；

例句
...a speech in which he affirmed a commitment to lower taxes...
他作出减税承诺的演讲

his

pron. (he的所有格)他的，(he的物主代词)他的东西；det. 属于他的，用于名词前，自己的，指某人的，上帝的；

例句
He claimed that his parents had abandoned him...
他声称父母遗弃了他。

voice

n. 嗓音，（动词的）语态，呼声，发言权，歌唱才能；vt. 发音，表达，说出，给（一部乐曲）配上声部，给（如管风琴音管）整声；

例句
Her voice is passionate. Her eyes are ablaze.
她声音激昂，目光如火。