契約自由原則為契約法之基本原理，包含缔約自由，相對人選擇自由，内容自由，變更或廢棄之自由，及方式自由等。基於契約自由原則，當事人可依其自由意思決定是否與他方缔結契約，決定契約之内容。但在高度工業化之現代社會，因為資本主義興起，資本集中於少數資本家而形成經濟上強勢（企業經營者）與經濟上弱勢（勞工•消費者）對立之架構。勞工•消費者於缔結契約之際，難以與具經濟上優勢地位之企業相抗衡，以致不能對等商議契約之内容，極易形成對消費者不公平之契約。為保護居經濟上地位弱勢之勞工•消費者，對於企業所能主張之契約自由原則受到一些限制，其中針對屬於獨占事業又為民生所必需者，限制其缔約之自由，令其有應消費者之請求與其缔結契約之義務。此舉形式上看似限制部分契約當事人之契約自由，但其目的乃係為達成契約的實質正義。就長程運輸或跨國境之運輸而言，民用航空運輸業具有事實上之獨占性及為民生所必需，因此當旅客提出訂立契約之請求時，除非有得以拒載之正當事由外，民用航空運輸業即有與其缔約之義務。

The principle of freedom of contract is the basic principle of the contract law. Through freedom of contract, individuals possess a general freedom to choose with whom to contract, whether to contract or not, and on which terms to contract. But in capitalist scciety a laborer/consumer can not compete with the trader under the principle of freedom of contract. For protecting the right of the laborer/consumer, we make some limitations to restrict the principle of freedom of contract. One of those is that we forced the trader to contract with a consumer who asks for contracting. Thus the trader can not choose with whom to contract.