With all the points discussions wrt the DC, there hasn't been one of these discussions for awhile. First of all, the OP can look forward to a lifetime of wonderful vacations and, while I wouldn't recommend buying something jointly while in a relationship that doesn't have a legal commitment, I applaud him for his sense of family and, at 22, recognizing the benefits derived from such travel with family and/or friends.
Although we didn't discover timesharing until a few years ago, when my kids were a little bit older than you, I can tell you that for many years we traveled as three generations, so when I calculated my costs I looked at the cost of at least 2 hotel rooms. As for MF's, for me they are about a wash in savings while away. When you figure in the cost of breakfast out (and the annoyance of starting off the day that way rather than the ease of breakfast on the balcony), snacks/drinks by the pool or beach and the cost of not eating out for the 2 nights that we generally like to grill, it pretty much balances out the annual MF's if you have 5 people or so to factor in. So personally my break even point is how many years of renting it would take to offset my (thankfully) resale purchases.
There are a few other considerations- owning means I don't have to look for rentals or have the insecurity of renting from a stranger or the cost of booking through Marriott, etc.. More importantly, if you own it, you'll use it- I don't think about whether I want to spend X$$'s on a trip or on something else. I love the fact that it forces us to take a break.
Another perk of ownership- access to Getaways. Ownership is more than the dollars and cents though; it is about quality time spent with those you love. Let me share a poignantly personal story: four years ago my parents celebrated their 60th Ann.. In anticipation of that wonderful event, we had booked a trip the beginning of January, when my daughter would be on intersession and be able to go. Their anniversary was the day before Thanksgiving. Anyway, because of the availability of Getaways, we decided to take my parents to Cancun for a week in mid November, just the four of us, as a pre-anniversary celebration. I picked up a great Getaway at the Royal Sands and, quite honestly, if not for that we probably wouldn't have done it, since we were still saddled down with tuition, etc.. We had a wonderful week, enjoying each other, the beach, dinners, shopping for my Mom and the ruins for my Dad, with memories we will forever cherish. Shockingly, we lost my Dad a month later, so that impromptu vacation turned out to be more memorable than we anticipated.
Because of the intangible aspects of ownership, the money spent although substantial is insignificant- the memories are truly priceless. You may not be able to put a dollar and cents figure on it, but it certainly does tip the scales.
Looks like those intangible aspects are enough justification for ownership for a lot of us!!