What does make an aspect like "strong","medium","loose" ?
Does this make aspect strong or weak ?
Or is just timing ?

I'd say a loose aspect is one that is above 5° of orb. For the JoS, tight aspects are under 2° orb. Different aspects also have different maximum orbs. Sextiles have 6° maximum while squares have up to 9°. So a 5° orb sextile is quite weak but a square of this orb can be strongly felt.

There goes a lot in what determines if an aspect is strong or weak. First, look at the orb. All tight aspects are usually strong. Also, look at the planets involved. If an aspect involves a chart ruler, it is often very strong even if the orb is loose.

If the planets are themselves strong, say with being in their home sign or exalted sign, the aspects will be stronger. Aspects involving angular houses (1st, 4th, 7th and 10th) are also stronger than those involving cadent houses (3rd, 6th, 9th, 12th).