1. A method of generating responses in a dialog system in response to user input in a current conversation session, comprising: collecting original user phrase data comprising the user input from one or more past conversation sessions between a first user only and the dialog system to form phrase models in a training phase using the first user's own phrases for use by a response generator in response to a query by the first user; using the phrase models at a semantic level to use wordings in the response that utilize the first user's own phrases and to identify constraint-carrying phrases from the first user's utterances; determining constraints expressed in each user utterance in the past conversation sessions using one or more constraint identifiers; forming a pool of user examples instructing the system on how to lexicalize the constraints to form lexicalized constraints; extracting one or more extracted constraints, each comprising a constraint-value pair, from the user input; generalizing the extracted constraints into phrase patterns using the lexicalized constraints to produce general phrase patterns; constructing a response database consisting of the original phrase data and the general phrase patterns; transmitting a set of filled constraints to the response generator component from the user input; and selecting at least one phrase from the response database that optimally matches the set of filled constraints to form a response utilizing the first user's own phrases.