～せいで — Because Of (And I Blame It)

電車が遅れたせいで、会議に遅刻した。

Densha ga okureta sei de, kaigi ni chikoku shita.

Because the train was late, I was late for the meeting.

Verb past 遅れた + せいで blames the delayed train for the bad result.

君のせいで計画が台無しだ。

Kimi no sei de keikaku ga dainashi da.

Because of you, the plan is ruined.

Noun 君 + の + せいで places blame directly on the listener.

暑いせいで、よく眠れなかった。

Atsui sei de, yoku nemurenakatta.

Because it was hot, I couldn't sleep well.

い-adj 暑い + せいで attributes the poor sleep to the heat.