Relationships pass through different stages. Initially there is the infatuation or the in love stage when partners learn about each other, explore new things both sexually and withing the relationship (they may be sexually intimate more often)
With time, the partners become used to each other, do not try to impress one another, feel a sense of security/complacence, etc) This stage may be causing boredom.
To have a romantic relationship all of the time is not realistic but to have romance in the relationship is needed. Spontaneity plays a big role into it, communication and compromise.
If his behavior is bothersome to you, address it seriously (sit him down and tell him specifically how it is affecting you. Guys do not think too much about it and unless brought to their attention may ignore the situation. Think of things to do to rekindle the spark.