Well, lavar is not reflexive for itself, you can wash your car (Lavar el coche).
The verbs turn into reflexive -when the action affects directly the person(s) who is/are performing it-. Say you wash your hands, the action of washing is performed by you and affecting you. Say you sit in the sofa, the action of sitting is performed by you, and also the consequences affect just you.
This is applicable to all I you we they he she it. If a woman (she) performs an action that affects directly and only her, again reflexive. If a group (they) performs an actions that affects directly themselves, same story. Etc, etc.
In those situations the conjugation changes.
Yo me lavo las manos
Tu te lavas las manos
El se lava las manos
Nosotros nos lavamos las manos
Vosotros os lavais las manos
Ellos se lavan las manos
Now take going to bed as an example.
Yo me acuesto
Tu te acuestas
El se acuesta
Nosotros nos acostamos
Vosotros os acostais
Ellos se acuestan
If you see, the verb remains the same reffering to conjugation. What changes for reflexive form is that, according to the case, 'me', 'te', 'se', 'nos', 'os', or 'se', will be added.
Hope its clear,
Alejo.
REF.: I'm a Spanish Teacher.