ことだし(And) so, Because, Since

Both express cause, but the stance is different. ことだし frames the reason as an accepted situation that makes the conclusion feel reasonable or timely, often used when gently proposing or deciding something, as in 時間もないことだし、先に始めよう ('Since we don’t have much time, let’s start first.'). ので is a neutral causal connector focusing on objective explanation, commonly used to state factual reasons without added nuance, as in 雨が降っているので、試合は中止です ('Because it is raining, the game is canceled.').

Both suggest that a result follows naturally, but the viewpoint is different. ことだし presents a reason as a settled circumstance that makes a decision reasonable at the moment, often used for gentle justification or proposals, as in 雨も降っていることだし、今日は家にいよう ('Since it’s raining anyway, let’s stay home today.'). べくして is a formal, written expression meaning 'bound to' or 'inevitably,' describing an outcome that follows naturally from prior conditions, often with hindsight, as in 彼は失敗すべくして失敗した ('He failed as was inevitable.').

Both give reasons, but the structure and emphasis differ. ことだし presents a reason as a settled or obvious circumstance that naturally justifies a decision, often sounding calm and explanatory, as in 雨も降っていることだし、今日は家にいよう ('Since it’s raining anyway, let’s stay home today.'). By contrast, し～し lists multiple reasons side by side, emphasizing accumulation rather than inevitability, as in 雨だし寒いし、出かけたくない ('It’s raining and cold, so I don’t want to go out.').

Both give reasons based on circumstances, but the nuance and formality differ. こととて is a formal, written expression used to justify an outcome by presenting the reason as unavoidable or understandable, often in apologies or explanations, as in 不慣れなこととて、ご迷惑をおかけしました ('As I am inexperienced, I caused you trouble.'). ことだし presents a reason as a known or settled situation that makes a decision feel reasonable or timely, and is common in spoken language, as in 雨も降っていることだし、今日は家にいよう ('Since it is raining anyway, let’s stay home today.').