Does not doing something mustahabb mean falling into makrooh?
Question

                If something is mustahab is doing it's opposite makrooh?.
Praise be to Allah.
    Not doing something mustahabb does not mean that one falls 
    into doing something makrooh in principle, because what is mustahabb comes 
    under the heading of commands and what is makrooh comes under the heading of 
    prohibitions. The word mustahabb refers to something that is enjoined in 
    Islam but is not absolutely binding, such as Duha prayer. The word makrooh 
    refers to something that is forbidden by Islam, but not in a manner that is 
    absolutely binding, such as the prohibition on women following funeral 
    processions. Umm Atiyyah (may Allah be pleased with her) said: We were 
    forbidden to follow funeral processions, but not strictly so. 

    Narrated by al-Bukhaari, 1219; Muslim, 938 

    The one who does something that is mustahabb will be 
    rewarded, and the one who does something that is makrooh is not sinning. 

    Based on that, it cannot be said that the one who does not 
    pray Duha has done something makrooh, just as it cannot be said that the one 
    who does not raise his hands when moving from one posture to another in the 
    prayer has done something makrooh. 

    Ibn Daqeeq al-Eid (may Allah have mercy on him) said: 

    We should note the difference between saying that it is 
    mustahabb to do such and such and saying that it is makrooh not to do it. 
    The one does not imply the other. It may be mustahabb to do a thing, but not 
    doing it is not makrooh, such as Duha prayer, for example, and many naafil 
    acts of worship. 

    End quote from Ihkaam al-Ahkaam Sharh ‘Umdat al-Ahkaam, 
    p. 19 

    Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: 

    Not doing something mustahabb is not necessarily makrooh. 

    End quote from Fath al-Baari, 11/17 

    Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have 
    mercy on him) said:

    Not doing something that is Sunnah does not necessarily mean 
    that one has fallen into makrooh. Hence if a person does not raise his hands 
    in prayer when bowing, he is not doing something that is makrooh. 

    This is a basic principle: not doing something mustahabb does 
    not necessarily mean falling into makrooh. 

    End quote from al-Sharh al-Mumti‘ ‘ala Zaad al-Mustaqni‘, 
    4/358 

    And Allah knows best.