Bataille says that identification of the self is, or should be, put in terms of communication with the unknowable "other." The self is therefore, not some entity isolated from the external world. In Bataille's thinking, the self is not an entity of sovereign identity. The subjectivity of the self is not defined and neatly encapsulated in a "self"-contained identity and isolated consciousness. The self is therefore unknowable in the way "I" would try to mentally bend back and conceive of myself as a whole self. Likewise, I can not fully know the "other" as a singular entity. Bataille is trying to avoid the Hegelian dialectic of self - other = union and/or subject - object = fusion. Instead, Bataille tries to put things in terms of scission (separation) and interaction as processes.
But, this creates a paradox of subjectivity. We need the notions of self and other in order to then even think about dissolving a self-other union (via love or some relationship). If the self can not know the other, how can love be intended towards or extended to the other? The answer Bataille provides is to consider the self, not as some closed entity, separate from the world of others. Rather, the subject who loves an "other" opens himself up to that other. Subjectivity is this opening up to the other in communication. The subject/self essentially loses itself in the movement of transgression/communication. This loss of self is akin to death; the self becomes movement.
This losing one's self is like a sacrifice (hence, the comparison to death - of the self). In Bataille's thinking, the loving encounter is this process of a self/subject losing his self in becoming a process of communication. In other words, love is not an exchange between two selves (subject and object) in which one or the other is subjected to the other. Rather, it is a process of communication in which (if equally reciprocated, if that is possible) each self ceases to be selves, becoming a process. Love is an experience of ecstasy which means joy but also means to be outside of one's self. It is a risk because this articulation of love means that the lover abandons his "self" in order to open up to the other as a process of interaction. His self is destroyed and/or sacrificed. It is similar to a sacred ritual sacrifice. This is why there is a notion of extreme vulnerability in genuine love; the self is risking him'self' completely.